Develop Reference

Reduce font-size when long word overflows horizontally - javascript

I have long titles that sometimes slightly overflow the window on small screens, so I made a script that reduces the font-size of the title until it fits its container only when the title overflows its container… It works but I'm sure a lighter solution could be found.
function adaptFontSize(titles) {
$(titles).each(function() {
var title = $(this),
reducedFontSize,
counter = 0;
if (title[0].scrollWidth <= title.innerWidth()) { // if title <= container
title.css('font-size', ''); // remove previously added font-size
}
while (title[0].scrollWidth > title.innerWidth() && counter < 20) { // while title > container
var fontSize = reducedFontSize || parseInt(title.css('font-size')); // current title font-size
reducedFontSize = Math.floor((fontSize - fontSize / 20) * 100) / 100; // reduce the font-size
title.css('font-size', reducedFontSize + 'px'); // apply the reduced font-size
counter++ // counter to limit the loop in case title[0].scrollWidth is not supported
}
});
}
or here's a JSFiddle: https://jsfiddle.net/Raoulito/pek7b3mr/12/
My problem comes when I try to apply the script on $(window).resize(). It actually works but I can feel that the process is heavy. It has to work in both ways:
reduce the font-size if the title's too wide,
increase the font-size when the window gets resized wider, until the title reaches its original font-size.
Here are some of steps I tried but did not manage to achieve:
that it does not fire the function all the way through at each pixel the window is resized, but rather wait until it stops resizing.
when the window gets resized wider, only target—among the titles which are smaller than the window—the ones that have already been resized and might need to get bigger again.
So I'm looking for a way to fix these, or alternatively for a way of reaching this in a much more efficient way.

You can use the VW unit.
1vw equals 1% of the view width, so it's a simple css only solution which works in all modern browsers.
https://jsfiddle.net/pek7b3mr/8/
body {
margin: 1em;
font-family: sans-serif;
}
div {
background-color: yellow;
font-size: 5vw;
margin-bottom: 1rem;
}
<div>ABCDEFGHIJKLMNOPQRSTUVWXYZ 012356789</div>

Related

I'm having a hard time getting my head around font scaling.
I currently have a website with a body font-size of 100%. 100% of what though? This seems to compute out at 16 pixels.
I was under the impression that 100% would somehow refer to the size of the browser window, but apparently not because it's always 16 pixels whether the window is resized down to a mobile width or full-blown widescreen desktop.
How can I make the text on my site scale in relation to its container? I tried using em, but this doesn't scale either.
My reasoning is that things like my menu become squished when you resize, so I need to reduce the px font-size of .menuItem among other elements in relation to the width of the container. (For example, in the menu on a large desktop, 22px works perfectly. Move down to tablet width and 16px is more appropriate.)
I'm aware I can add breakpoints, but I really want the text to scale as well as having extra breakpoints, otherwise, I'll end up with hundreds of breakpoints for every 100pixels decrease in width to control the text.

If the container is not the body, CSS Tricks covers all of your options in Fitting Text to a Container.
If the container is the body, what you are looking for is Viewport-percentage lengths:
The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly. However, when the value of overflow on the root element is auto, any scroll bars are assumed not to exist.
The values are:
vw (% of the viewport width)
vh (% of the viewport height)
vi (1% of the viewport size in the direction of the root element's inline axis)
vb (1% of the viewport size in the direction of the root element's block axis)
vmin (the smaller of vw or vh)
vmax (the larger or vw or vh)
1 v* is equal to 1% of the initial containing block.
Using it looks like this:
p {
font-size: 4vw;
}
As you can see, when the viewport width increases, so do the font-size, without needing to use media queries.
These values are a sizing unit, just like px or em, so they can be used to size other elements as well, such as width, margin, or padding.
Browser support is pretty good, but you'll likely need a fallback, such as:
p {
font-size: 16px;
font-size: 4vw;
}
Check out the support statistics: http://caniuse.com/#feat=viewport-units.
Also, check out CSS-Tricks for a broader look: Viewport Sized Typography
Here's a nice article about setting minimum/maximum sizes and exercising a bit more control over the sizes: Precise control over responsive typography
And here's an article about setting your size using calc() so that the text fills the viewport: http://codepen.io/CrocoDillon/pen/fBJxu
Also, please view this article, which uses a technique dubbed 'molten leading' to adjust the line-height as well. Molten Leading in CSS

But what if the container is not the viewport (body)?
This question is asked in a comment by Alex under 2507rkt3's answer.
That fact does not mean vw cannot be used to some extent to size for that container. Now to see any variation at all one has to be assuming that the container in some way is flexible in size. Whether through a direct percentage width or through being 100% minus margins. The point becomes "moot" if the container is always set to, let's say, 200px wide--then just set a font-size that works for that width.
Example 1
With a flexible width container, however, it must be realized that in some way the container is still being sized off the viewport. As such, it is a matter of adjusting a vw setting based off that percentage size difference to the viewport, which means taking into account the sizing of parent wrappers. Take this example:
div {
width: 50%;
border: 1px solid black;
margin: 20px;
font-size: 16px;
/* 100 = viewport width, as 1vw = 1/100th of that
So if the container is 50% of viewport (as here)
then factor that into how you want it to size.
Let's say you like 5vw if it were the whole width,
then for this container, size it at 2.5vw (5 * .5 [i.e. 50%])
*/
font-size: 2.5vw;
}
Assuming here the div is a child of the body, it is 50% of that 100% width, which is the viewport size in this basic case. Basically, you want to set a vw that is going to look good to you. As you can see in my comment in the above CSS content, you can "think" through that mathematically with respect to the full viewport size, but you don't need to do that. The text is going to "flex" with the container because the container is flexing with the viewport resizing. Here's an example of two differently sized containers.
Example 2
You can help ensure viewport sizing by forcing the calculation based off that. Consider this example:
html {width: 100%;} /* Force 'html' to be viewport width */
body {width: 150%; } /* Overflow the body */
div {
width: 50%;
border: 1px solid black;
margin: 20px;
font-size: 16px;
/* 100 = viewport width, as 1vw = 1/100th of that
Here, the body is 150% of viewport, but the container is 50%
of viewport, so both parents factor into how you want it to size.
Let's say you like 5vw if it were the whole width,
then for this container, size it at 3.75vw
(5 * 1.5 [i.e. 150%]) * .5 [i.e. 50%]
*/
font-size: 3.75vw;
}
The sizing is still based off viewport, but is in essence set up based off the container size itself.
Should the Size of the Container Change Dynamically...
If the sizing of the container element ended up changing dynamically its percentage relationship either via #media breakpoints or via JavaScript, then whatever the base "target" was would need recalculation to maintain the same "relationship" for text sizing.
Take example #1 above. If the div was switched to 25% width by either #media or JavaScript, then at the same time, the font-size would need to adjust in either the media query or by JavaScript to the new calculation of 5vw * .25 = 1.25. This would put the text size at the same size it would have been had the "width" of the original 50% container been reduced by half from viewport sizing, but has now been reduced due to a change in its own percentage calculation.
A Challenge
With the CSS calc() function in use, it would become difficult to adjust dynamically, as that function does not work for font-size purposes at this time. So you could not do a pure CSS adjustment if your width is changing on calc(). Of course, a minor adjustment of width for margins may not be enough to warrant any change in font-size, so it may not matter.

Solution with SVG:
.resizeme {
resize: both;
margin: 0;
padding: 0;
height: 75px;
width: 500px;
background-color: lightblue;
overflow: hidden;
}
<div class="resizeme">
<svg
width="100%"
height="100%"
viewBox="0 0 500 75"
preserveAspectRatio="xMinYMid meet"
style="background-color:green"
xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink"
>
<text
x="0"
y="75"
font-size="75"
fill="black"
>█Resize This█</text>
</svg>
</div>
Solution with SVG and text-wrapping using foreignObject:
.resizeme {
resize: both;
margin: 0;
padding: 0;
height: 200px;
width: 500px;
background-color: lightblue;
overflow: hidden;
}
<div class="resizeme">
<svg
width="100%"
height="100%"
viewBox="0 0 500 200"
preserveAspectRatio="xMinYMin meet"
>
<foreignObject width="100%" height="100%" xmlns="http://www.w3.org/1999/xhtml">
<div xmlns="http://www.w3.org/1999/xhtml" style="background-color:lightgreen;">
<h1>heading</h1>
<p>Resize the blue box.</p>
</div>
</foreignObject>
</svg>
</div>

In one of my projects I use a "mixture" between vw and vh to adjust the font size to my needs, for example:
font-size: calc(3vw + 3vh);
I know this doesn't answer the OP's question, but maybe it can be a solution to anyone else.

Pure-CSS solution with calc(), CSS units and math
This is precisely not what OP asks, but may make someone's day. This answer is not spoon-feedingly easy and needs some researching on the developer end.
I came finally to get a pure-CSS solution for this using calc() with different units. You will need some basic mathematical understanding of formulas to work out your expression for calc().
When I worked this out, I had to get a full-page-width responsive header with some padding few parents up in DOM. I'll use my values here, replace them with your own.
To mathematics
You will need:
Nicely adjusted ratio in some viewport. I used 320 pixels, thus I got 24 pixels high and 224 pixels wide, so the ratio is 9.333... or 28 / 3
The container width, I had padding: 3em and full width so this got to 100wv - 2 * 3em
X is the width of container, so replace it with your own expression or adjust the value to get full-page text. R is the ratio you will have. You can get it by adjusting the values in some viewport, inspecting element width and height and replacing them with your own values. Also, it is width / heigth ;)
x = 100vw - 2 * 3em = 100vw - 6em
r = 224px/24px = 9.333... = 28 / 3
y = x / r
= (100vw - 6em) / (28 / 3)
= (100vw - 6em) * 3 / 28
= (300vw - 18em) / 28
= (75vw - 4.5rem) / 7
And bang! It worked! I wrote
font-size: calc((75vw - 4.5rem) / 7)
to my header and it adjusted nicely in every viewport.
But how does it work?
We need some constants up here. 100vw means the full width of viewport, and my goal was to establish full-width header with some padding.
The ratio. Getting a width and height in one viewport got me a ratio to play with, and with ratio I know what the height should be in other viewport width. Calculating them with hand would take plenty of time and at least take lots of bandwidth, so it's not a good answer.
Conclusion
I wonder why no-one has figured this out and some people are even telling that this would be impossible to tinker with CSS. I don't like to use JavaScript in adjusting elements, so I don't accept JavaScript (and forget about jQuery) answers without digging more. All in all, it's good that this got figured out and this is one step to pure-CSS implementations in website design.
I apologize of any unusual convention in my text, I'm not native speaker in English and am also quite new to writing Stack Overflow answers.
It should also be noted that we have evil scrollbars in some browsers. For example, when using Firefox I noticed that 100vw means the full width of viewport, extending under scrollbar (where content cannot expand!), so the fullwidth text has to be margined carefully and preferably get tested with many browsers and devices.

There is a big philosophy for this issue.
The easiest thing to do would be to give a certain font-size to body (I recommend 10), and then all the other element would have their font in em or rem.
I'll give you an example to understand those units.
Em is always relative to its parent:
body{font-size: 10px;}
.menu{font-size: 2em;} /* That means 2*10 pixels = 20 pixels */
.menu li{font-size: 1.5em;} /* That means 1.5*20 pixels = 30 pixels */
Rem is always relative to body:
body{font-size: 10px;}
.menu{font-size: 2rem;} /* That means 2*10 pixels = 20 pixels */
.menu li{font-size: 1.5rem;} /* that means 1.5*10 pixels = 15 pixels */
And then you could create a script that would modify font-size relative to your container width.
But this isn't what I would recommend. Because in a 900 pixels width container for example you would have a p element with a 12 pixels font-size let's say. And on your idea that would become an 300 pixels wide container at 4 pixels font-size. There has to be a lower limit.
Other solutions would be with media queries, so that you could set font for different widths.
But the solutions that I would recommend is to use a JavaScript library that helps you with that. And fittext.js that I found so far.

Here is the function:
document.body.setScaledFont = function(f) {
var s = this.offsetWidth, fs = s * f;
this.style.fontSize = fs + '%';
return this
};
Then convert all your documents child element font sizes to em's or %.
Then add something like this to your code to set the base font size.
document.body.setScaledFont(0.35);
window.onresize = function() {
document.body.setScaledFont(0.35);
}
http://jsfiddle.net/0tpvccjt/

There is a way to do this without JavaScript!
You can use an inline SVG image. You can use CSS on an SVG if it is inline. You have to remember that using this method means your SVG image will respond to its container size.
Try using the following solution...
HTML
<div>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 360.96 358.98" >
<text>SAVE $500</text>
</svg>
</div>
CSS
div {
width: 50%; /* Set your container width */
height: 50%; /* Set your container height */
}
svg {
width: 100%;
height: auto;
}
text {
transform: translate(40px, 202px);
font-size: 62px;
fill: #000;
}
Example:
https://jsfiddle.net/k8L4xLLa/32/
Want something more flashy?
SVG images also allow you to do cool stuff with shapes and junk. Check out this great use case for scalable text...
https://jsfiddle.net/k8L4xLLa/14/

CSS Container Queries
A late-2022 addition to the CSS feature set makes scaling font size with containers straightforward.
Container queries come with a new set of CSS units cqw/cqh (container query width/height). To use them you need to set the container-type property on the parent element whose size you want to use. Minimal example:
<div>
<p>Lorem ipsum dolor sit amet</p>
</div>
<style>
div {
container-type: inline-size;
}
p {
font-size: 5cqw;
}
</style>
The font size will increase smoothly as the container grows. At 1000px container width, the p font size will be 1000px / 100 * 5 = 50px.
container-type can be size or inline-size. size tracks both height and width of the container which allows you to use both cqw and cqh.
Most of the time on the web, heights are calculated based on content and you only specify the width. To save the browser some work, you'll generally want to set container-type: inline-size; so the browser only tracks the inline dimension which is usually width (unless you set writing-mode to vertical).
Browser support for container queries has grown rapidly in the 2nd half of 2022 and currently stands at 75% (2023-01-01).

This may not be super practical, but if you want a font to be a direct function of the parent, without having any JavaScript that listens/loops (interval) to read the size of the div/page, there is a way to do it. Iframes.
Anything within the iframe will consider the size of the iframe as the size of the viewport. So the trick is to just make an iframe whose width is the maximum width you want your text to be, and whose height is equal to the maximum height * the particular text's aspect ratio.
Setting aside the limitation that viewport units can't also come along side parent units for text (as in, having the % size behave like everyone else), viewport units do provide a very powerful tool: being able to get the minimum/maximum dimension. You can't do that anywhere else - you can't say...make the height of this div be the width of the parent * something.
That being said, the trick is to use vmin, and to set the iframe size so that [fraction] * total height is a good font size when the height is the limiting dimension, and [fraction] * total width when the width is the limiting dimension. This is why the height has to be a product of the width and the aspect ratio.
For my particular example, you have
.main iframe{
position: absolute;
top: 50%;
left: 50%;
width: 100%;
height: calc(3.5 * 100%);
background: rgba(0, 0, 0, 0);
border-style: none;
transform: translate3d(-50%, -50%, 0);
}
The small annoyance with this method is that you have to manually set the CSS of the iframe. If you attach the whole CSS file, that would take up a lot of bandwidth for many text areas. So, what I do is attach the rule that I want directly from my CSS.
var rule = document.styleSheets[1].rules[4];
var iDoc = document.querySelector('iframe').contentDocument;
iDoc.styleSheets[0].insertRule(rule.cssText);
You can write small function that gets the CSS rule / all CSS rules that would affect the text area.
I cannot think of another way to do it without having some cycling/listening JavaScript. The real solution would be for browsers to provide a way to scale text as a function of the parent container and to also provide the same vmin/vmax type functionality.
JSFiddle:
https://jsfiddle.net/0jr7rrgm/3/
(click once to lock the red square to the mouse, and click again to release)
Most of the JavaScript in the fiddle is just my custom click-drag function.

Using vw, em & co. works for sure, but IMO it always needs a human's touch for fine-tuning.
Here's a script I just wrote based on #tnt-rox' answer that tries to automatize that human's touch:
$('#controller').click(function(){
$('h2').each(function(){
var
$el = $(this),
max = $el.get(0),
el = null
;
max =
max
? max.offsetWidth
: 320
;
$el.css({
'font-size': '1em',
'display': 'inline',
});
el = $el.get(0);
el.get_float = function(){
var
fs = 0
;
if (this.style && this.style.fontSize) {
fs = parseFloat(this.style.fontSize.replace(/([\d\.]+)em/g, '$1'));
}
return fs;
};
el.bigger = function(){
this.style.fontSize = (this.get_float() + 0.1) + 'em';
};
while (el.offsetWidth < max) {
el.bigger();
}
// Finishing touch.
$el.css({
'font-size': ((el.get_float() -0.1) +'em'),
'line-height': 'normal',
'display': '',
});
}); // end of (each)
}); // end of (font scaling test)
div {
width: 50%;
background-color: tomato;
font-family: 'Arial';
}
h2 {
white-space: nowrap;
}
h2:nth-child(2) {
font-style: italic;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<input type="button" id="controller" value="Apply" />
<div>
<h2>Lorem ipsum dolor</h2>
<h2>Test String</h2>
<h2>Sweet Concatenation</h2>
<h2>Font Scaling</h2>
</div>
It basically reduces the font-size to 1em and then starts incrementing by 0.1 until it reaches maximum width.
JSFiddle

Use CSS Variables
No one has mentioned CSS variables yet, and this approach worked best for me, so:
Let's say you've got a column on your page that is 100% of the width of a mobile user's screen, but has a max-width of 800px, so on desktop there's some space on either side of the column. Put this at the top of your page:
<script> document.documentElement.style.setProperty('--column-width', Math.min(window.innerWidth, 800)+'px'); </script>
And now you can use that variable (instead of the built-in vw unit) to set the size of your font. E.g.
p {
font-size: calc( var(--column-width) / 100 );
}
It's not a pure CSS approach, but it's pretty close.

100% is relative to the base font size, which, if you haven't set it, would be the browser's user-agent default.
To get the effect you're after, I would use a piece of JavaScript code to adjust the base font size relative to the window dimensions.

Artistically, if you need to fit two or more lines of text within the same width regardless of their character count then you have nice options.
It's best to find a dynamical solution so whatever text is entered we end up with a nice display.
Let's see how we may approach.
var els = document.querySelectorAll(".divtext"),
refWidth = els[0].clientWidth,
refFontSize = parseFloat(window.getComputedStyle(els[0],null)
.getPropertyValue("font-size"));
els.forEach((el,i) => el.style.fontSize = refFontSize * refWidth / els[i].clientWidth + "px")
#container {
display: inline-block;
background-color: black;
padding: 0.6vw 1.2vw;
}
.divtext {
display: table;
color: white;
font-family: impact;
font-size: 4.5vw;
}
<div id="container">
<div class="divtext">THIS IS JUST AN</div>
<div class="divtext">EXAMPLE</div>
<div class="divtext">TO SHOW YOU WHAT</div>
<div class="divtext">YOU WANT</div>
</div>
All we do is to get the width (els[0].clientWidth) and the font size (parseFloat(window.getComputedStyle(els[0],null).getPropertyValue("font-size"))) of the first line as a reference and then just calculate the subsequent lines font size accordingly.

This web component changes the font size so the inner text width matches the container width. Check the demo.
You can use it like this:
<full-width-text>Lorem Ipsum</full-width-text>

You may be you looking for something like this:
http://jsfiddle.net/sijav/dGsC9/4/
http://fiddle.jshell.net/sijav/dGsC9/4/show/
I have used flowtype, and it's working great (however it's JavaScript and not a pure CSS solution):
$('body').flowtype({
minFont: 10,
maxFont: 40,
minimum: 500,
maximum: 1200,
fontRatio: 70
});

I've prepared a simple scale function using CSS transform instead of font-size. You can use it inside of any container, you don't have to set media queries, etc. :)
Blog post:
Full width CSS & JS scalable header
The code:
function scaleHeader() {
var scalable = document.querySelectorAll('.scale--js');
var margin = 10;
for (var i = 0; i < scalable.length; i++) {
var scalableContainer = scalable[i].parentNode;
scalable[i].style.transform = 'scale(1)';
var scalableContainerWidth = scalableContainer.offsetWidth - margin;
var scalableWidth = scalable[i].offsetWidth;
scalable[i].style.transform = 'scale(' + scalableContainerWidth / scalableWidth + ')';
scalableContainer.style.height = scalable[i].getBoundingClientRect().height + 'px';
}
}
Working demo:
https://codepen.io/maciejkorsan/pen/BWLryj

Inside your CSS, try adding this at the bottom changing the 320 pixels width for wherever your design starts breaking:
#media only screen and (max-width: 320px) {
body { font-size: 1em; }
}
Then give the font-size in "px" or "em" as you wish.

Try http://simplefocus.com/flowtype/. This is what I use for my sites, and it has worked perfectly.

My own solution, jQuery-based, works by gradually increasing the font size until the container gets a big increase in height (meaning it got a line break).
It's pretty simple, but works fairly well, and it is very easy to use. You don't have to know anything about the font being used, everything is taken care of by the browser.
You can play with it on http://jsfiddle.net/tubededentifrice/u5y15d0L/2/
The magic happens here:
var setMaxTextSize=function(jElement) {
// Get and set the font size into data for reuse upon resize
var fontSize=parseInt(jElement.data(quickFitFontSizeData)) || parseInt(jElement.css("font-size"));
jElement.data(quickFitFontSizeData, fontSize);
// Gradually increase font size until the element gets a big increase in height (i.e. line break)
var i = 0;
var previousHeight;
do
{
previousHeight=jElement.height();
jElement.css("font-size", "" + (++fontSize) + "px");
}
while(i++ < 300 && jElement.height()-previousHeight < fontSize/2)
// Finally, go back before the increase in height and set the element as resized by adding quickFitSetClass
fontSize -= 1;
jElement.addClass(quickFitSetClass).css("font-size", "" + fontSize + "px");
return fontSize;
};

My problem was similar, but related to scaling text within a heading. I tried Fit Font, but I needed to toggle the compressor to get any results, since it was solving a slightly different problem, as was Text Flow.
So I wrote my own little plugin that reduced the font size to fit the container, assuming you have overflow: hidden and white-space: nowrap so that even if reducing the font to the minimum doesn't allow showing the full heading, it just cuts off what it can show.
(function($) {
// Reduces the size of text in the element to fit the parent.
$.fn.reduceTextSize = function(options) {
options = $.extend({
minFontSize: 10
}, options);
function checkWidth(em) {
var $em = $(em);
var oldPosition = $em.css('position');
$em.css('position', 'absolute');
var width = $em.width();
$em.css('position', oldPosition);
return width;
}
return this.each(function(){
var $this = $(this);
var $parent = $this.parent();
var prevFontSize;
while (checkWidth($this) > $parent.width()) {
var currentFontSize = parseInt($this.css('font-size').replace('px', ''));
// Stop looping if min font size reached, or font size did not change last iteration.
if (isNaN(currentFontSize) || currentFontSize <= options.minFontSize ||
prevFontSize && prevFontSize == currentFontSize) {
break;
}
prevFontSize = currentFontSize;
$this.css('font-size', (currentFontSize - 1) + 'px');
}
});
};
})(jQuery);

Try to use the fitText plugin, because Viewport sizes isn't the solution of this problem.
Just add the library:
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
And change font-size for correct by settings the coefficient of text:
$("#text_div").fitText(0.8);
You can set maximum and minimum values of text:
$("#text_div").fitText(0.8, { minFontSize: '12px', maxFontSize: '36px' });

Here is a pure CSS solution with the understanding that you admit breakpoints are necessary but also want text scaling:
I'm aware I can add breakpoints, but I really want the text to scale
as well as having extra breakpoints, otherwise....
Here is an approach using:
Custom properties
Media queries for breakpoints
clamp() (browser support in Feb 2022 is pretty good at 93%)
calc()
If one common scaling factor can be used to control ALL the text scaling within a container per screen max-width, all you need to do is scale a custom property per max-width, and apply this factor to 1 calculation.
A basic setup starts like this:
:root {
--scaling-factor: 1
}
.parent {
font-size: 30px
}
.largest {
font-size: clamp(60%, calc(var(--scaling-factor) * 100%), 100%);
}
.middle {
font-size: clamp(60%, calc(var(--scaling-factor) * 85%), 100%);
}
.smallest {
font-size: clamp(60%, calc(var(--scaling-factor) * 70%), 100%);
}
Then nest your media queries something like this (or whatever you need for your breakpoints):
#media (max-width: 1200px) {
:root {
--scaling-factor: 0.9
}
#media (max-width: 800px) {
:root {
--scaling-factor: 0.8
}
#media (max-width: 600px) {
:root {
--scaling-factor: 0.5 /* nope, because the font-size is floored at 60% thanks to clamp() */
}
}
}
}
This minimizes your media query markup.
Advantages
One custom property controls ALL scaling ... no need to add multiple declarations per media breakpoint
The use of clamp() sets a lower-limit on what the font-size should be, so you ensure your text is never too small (here the floor is 60% of the parent's font-size)
Please see this JSFiddle for a demo. Resize the window until at the smallest widths, the paragraphs are all the same font-size.

Always have your element with this attribute:
JavaScript: element.style.fontSize = "100%";
or
CSS: style = "font-size: 100%;"
When you go fullscreen, you should already have a scale variable calculated (scale > 1 or scale = 1). Then, on fullscreen:
document.body.style.fontSize = (scale * 100) + "%";
It works nicely with little code.

Take look at my code. It makes the font size smaller to fit whatever there.
But I think this doesn't lead to a good user experience
var containerWidth = $("#ui-id-2").width();
var items = $(".quickSearchAutocomplete .ui-menu-item");
var fontSize = 16;
items.each(function(){
// Displaying a value depends sometimes on your case. You may make it block or inline-table instead of inline-block or whatever value that make the div take overflow width.
$(this).css({"whiteSpace": "nowrap", "display": "inline-block"});
while ($(this).width() > containerWidth){
console.log("$(this).width()" + $(this).width() + "containerWidth" + containerWidth)
$(this).css("font-size", fontSize -= 0.5);
}
});

For dynamic text, this plugin is quite useful:
http://freqdec.github.io/slabText/
Simply add CSS:
.slabtexted .slabtext
{
display: -moz-inline-box;
display: inline-block;
white-space: nowrap;
}
.slabtextinactive .slabtext
{
display: inline;
white-space: normal;
font-size: 1em !important;
letter-spacing: inherit !important;
word-spacing: inherit !important;
*letter-spacing: normal !important;
*word-spacing: normal !important;
}
.slabtextdone .slabtext
{
display: block;
}
And the script:
$('#mydiv').slabText();

This worked for me:
I try to approximate font-size based on a width/height got from setting `font-size: 10px`. Basically, the idea is "if I have 20 pixels width and 11 pixels height with `font-size: 10px`, so what would it be the maximum font-size to math a container of 50 pixels width and 30 pixels height?"
The answer is a double proportion system:
{ 20:10=50:X, 11:10=30:Y } = { X= (10*50)/20, Y= (10*30)/11 }
Now X is a font-size that will match width, and Y is a font-size that will match height; take the smallest value
function getMaxFontSizeApprox(el){
var fontSize = 10;
var p = el.parentNode;
var parent_h = p.offsetHeight ? p.offsetHeight : p.style.pixelHeight;
if(!parent_h)
parent_h = 0;
var parent_w = p.offsetHeight ? p.offsetWidth : p.style.pixelWidth;
if(!parent_w)
parent_w = 0;
el.style.fontSize = fontSize + "px";
var el_h = el.offsetHeight ? el.offsetHeight : el.style.pixelHeight;
if(!el_h)
el_h = 0;
var el_w = el.offsetHeight ? el.offsetWidth : el.style.pixelWidth;
if(!el_w)
el_w = 0;
// 0.5 is the error on the measure that JavaScript does
// if the real measure had been 12.49 px => JavaScript would have said 12px
// so we think about the worst case when could have, we add 0.5 to
// compensate the round error
var fs1 = (fontSize*(parent_w + 0.5))/(el_w + 0.5);
var fs2 = (fontSize*(parent_h) + 0.5)/(el_h + 0.5);
fontSize = Math.floor(Math.min(fs1,fs2));
el.style.fontSize = fontSize + "px";
return fontSize;
}
NB: the argument of the function must be a span element or an element which is smaller than its parent, otherwise if children and parent have both the same width/height function will fail.

let textElement = document.getElementById('text1');
let parentElement = textElement.parentElement;
const parentClientHeight = parentElement.clientHeight;
const parentClientWidth = parentElement.clientWidth;
textElement.style.padding = "unset";
textElement.style.margin = "auto";
let fontSize = parentClientHeight;
let minFS = 3,
maxFS = fontSize;
while (fontSize != minFS) {
textElement.style.fontSize = `${fontSize}px`;
if (
parentElement.scrollHeight <= parentClientHeight &&
parentElement.scrollWidth <= parentClientWidth
) {
minFS = fontSize;
} else {
maxFS = fontSize;
}
fontSize = Math.floor((minFS + maxFS) / 2);
}
textElement.style.fontSize = `${minFS}px`;
<div style="height:200px; width:300px;">
<div id='text1'>
test
</div>
</div>

As a JavaScript fallback (or your sole solution), you can use my jQuery Scalem plugin, which lets you scale relative to the parent element (container) by passing the reference option.

In case it's helpful to anyone, most of the solutions in this thread were wrapping text into multiple lines, form e.
But then I found this, and it worked:
https://github.com/chunksnbits/jquery-quickfit
Example usage:
$('.someText').quickfit({max:50,tolerance:.4})

I'm having a hard time getting my head around font scaling.
I currently have a website with a body font-size of 100%. 100% of what though? This seems to compute out at 16 pixels.
I was under the impression that 100% would somehow refer to the size of the browser window, but apparently not because it's always 16 pixels whether the window is resized down to a mobile width or full-blown widescreen desktop.
How can I make the text on my site scale in relation to its container? I tried using em, but this doesn't scale either.
My reasoning is that things like my menu become squished when you resize, so I need to reduce the px font-size of .menuItem among other elements in relation to the width of the container. (For example, in the menu on a large desktop, 22px works perfectly. Move down to tablet width and 16px is more appropriate.)
I'm aware I can add breakpoints, but I really want the text to scale as well as having extra breakpoints, otherwise, I'll end up with hundreds of breakpoints for every 100pixels decrease in width to control the text.

If the container is not the body, CSS Tricks covers all of your options in Fitting Text to a Container.
If the container is the body, what you are looking for is Viewport-percentage lengths:
The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly. However, when the value of overflow on the root element is auto, any scroll bars are assumed not to exist.
The values are:
vw (% of the viewport width)
vh (% of the viewport height)
vi (1% of the viewport size in the direction of the root element's inline axis)
vb (1% of the viewport size in the direction of the root element's block axis)
vmin (the smaller of vw or vh)
vmax (the larger or vw or vh)
1 v* is equal to 1% of the initial containing block.
Using it looks like this:
p {
font-size: 4vw;
}
As you can see, when the viewport width increases, so do the font-size, without needing to use media queries.
These values are a sizing unit, just like px or em, so they can be used to size other elements as well, such as width, margin, or padding.
Browser support is pretty good, but you'll likely need a fallback, such as:
p {
font-size: 16px;
font-size: 4vw;
}
Check out the support statistics: http://caniuse.com/#feat=viewport-units.
Also, check out CSS-Tricks for a broader look: Viewport Sized Typography
Here's a nice article about setting minimum/maximum sizes and exercising a bit more control over the sizes: Precise control over responsive typography
And here's an article about setting your size using calc() so that the text fills the viewport: http://codepen.io/CrocoDillon/pen/fBJxu
Also, please view this article, which uses a technique dubbed 'molten leading' to adjust the line-height as well. Molten Leading in CSS

But what if the container is not the viewport (body)?
This question is asked in a comment by Alex under 2507rkt3's answer.
That fact does not mean vw cannot be used to some extent to size for that container. Now to see any variation at all one has to be assuming that the container in some way is flexible in size. Whether through a direct percentage width or through being 100% minus margins. The point becomes "moot" if the container is always set to, let's say, 200px wide--then just set a font-size that works for that width.
Example 1
With a flexible width container, however, it must be realized that in some way the container is still being sized off the viewport. As such, it is a matter of adjusting a vw setting based off that percentage size difference to the viewport, which means taking into account the sizing of parent wrappers. Take this example:
div {
width: 50%;
border: 1px solid black;
margin: 20px;
font-size: 16px;
/* 100 = viewport width, as 1vw = 1/100th of that
So if the container is 50% of viewport (as here)
then factor that into how you want it to size.
Let's say you like 5vw if it were the whole width,
then for this container, size it at 2.5vw (5 * .5 [i.e. 50%])
*/
font-size: 2.5vw;
}
Assuming here the div is a child of the body, it is 50% of that 100% width, which is the viewport size in this basic case. Basically, you want to set a vw that is going to look good to you. As you can see in my comment in the above CSS content, you can "think" through that mathematically with respect to the full viewport size, but you don't need to do that. The text is going to "flex" with the container because the container is flexing with the viewport resizing. Here's an example of two differently sized containers.
Example 2
You can help ensure viewport sizing by forcing the calculation based off that. Consider this example:
html {width: 100%;} /* Force 'html' to be viewport width */
body {width: 150%; } /* Overflow the body */
div {
width: 50%;
border: 1px solid black;
margin: 20px;
font-size: 16px;
/* 100 = viewport width, as 1vw = 1/100th of that
Here, the body is 150% of viewport, but the container is 50%
of viewport, so both parents factor into how you want it to size.
Let's say you like 5vw if it were the whole width,
then for this container, size it at 3.75vw
(5 * 1.5 [i.e. 150%]) * .5 [i.e. 50%]
*/
font-size: 3.75vw;
}
The sizing is still based off viewport, but is in essence set up based off the container size itself.
Should the Size of the Container Change Dynamically...
If the sizing of the container element ended up changing dynamically its percentage relationship either via #media breakpoints or via JavaScript, then whatever the base "target" was would need recalculation to maintain the same "relationship" for text sizing.
Take example #1 above. If the div was switched to 25% width by either #media or JavaScript, then at the same time, the font-size would need to adjust in either the media query or by JavaScript to the new calculation of 5vw * .25 = 1.25. This would put the text size at the same size it would have been had the "width" of the original 50% container been reduced by half from viewport sizing, but has now been reduced due to a change in its own percentage calculation.
A Challenge
With the CSS calc() function in use, it would become difficult to adjust dynamically, as that function does not work for font-size purposes at this time. So you could not do a pure CSS adjustment if your width is changing on calc(). Of course, a minor adjustment of width for margins may not be enough to warrant any change in font-size, so it may not matter.

Solution with SVG:
.resizeme {
resize: both;
margin: 0;
padding: 0;
height: 75px;
width: 500px;
background-color: lightblue;
overflow: hidden;
}
<div class="resizeme">
<svg
width="100%"
height="100%"
viewBox="0 0 500 75"
preserveAspectRatio="xMinYMid meet"
style="background-color:green"
xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink"
>
<text
x="0"
y="75"
font-size="75"
fill="black"
>█Resize This█</text>
</svg>
</div>
Solution with SVG and text-wrapping using foreignObject:
.resizeme {
resize: both;
margin: 0;
padding: 0;
height: 200px;
width: 500px;
background-color: lightblue;
overflow: hidden;
}
<div class="resizeme">
<svg
width="100%"
height="100%"
viewBox="0 0 500 200"
preserveAspectRatio="xMinYMin meet"
>
<foreignObject width="100%" height="100%" xmlns="http://www.w3.org/1999/xhtml">
<div xmlns="http://www.w3.org/1999/xhtml" style="background-color:lightgreen;">
<h1>heading</h1>
<p>Resize the blue box.</p>
</div>
</foreignObject>
</svg>
</div>

In one of my projects I use a "mixture" between vw and vh to adjust the font size to my needs, for example:
font-size: calc(3vw + 3vh);
I know this doesn't answer the OP's question, but maybe it can be a solution to anyone else.

Pure-CSS solution with calc(), CSS units and math
This is precisely not what OP asks, but may make someone's day. This answer is not spoon-feedingly easy and needs some researching on the developer end.
I came finally to get a pure-CSS solution for this using calc() with different units. You will need some basic mathematical understanding of formulas to work out your expression for calc().
When I worked this out, I had to get a full-page-width responsive header with some padding few parents up in DOM. I'll use my values here, replace them with your own.
To mathematics
You will need:
Nicely adjusted ratio in some viewport. I used 320 pixels, thus I got 24 pixels high and 224 pixels wide, so the ratio is 9.333... or 28 / 3
The container width, I had padding: 3em and full width so this got to 100wv - 2 * 3em
X is the width of container, so replace it with your own expression or adjust the value to get full-page text. R is the ratio you will have. You can get it by adjusting the values in some viewport, inspecting element width and height and replacing them with your own values. Also, it is width / heigth ;)
x = 100vw - 2 * 3em = 100vw - 6em
r = 224px/24px = 9.333... = 28 / 3
y = x / r
= (100vw - 6em) / (28 / 3)
= (100vw - 6em) * 3 / 28
= (300vw - 18em) / 28
= (75vw - 4.5rem) / 7
And bang! It worked! I wrote
font-size: calc((75vw - 4.5rem) / 7)
to my header and it adjusted nicely in every viewport.
But how does it work?
We need some constants up here. 100vw means the full width of viewport, and my goal was to establish full-width header with some padding.
The ratio. Getting a width and height in one viewport got me a ratio to play with, and with ratio I know what the height should be in other viewport width. Calculating them with hand would take plenty of time and at least take lots of bandwidth, so it's not a good answer.
Conclusion
I wonder why no-one has figured this out and some people are even telling that this would be impossible to tinker with CSS. I don't like to use JavaScript in adjusting elements, so I don't accept JavaScript (and forget about jQuery) answers without digging more. All in all, it's good that this got figured out and this is one step to pure-CSS implementations in website design.
I apologize of any unusual convention in my text, I'm not native speaker in English and am also quite new to writing Stack Overflow answers.
It should also be noted that we have evil scrollbars in some browsers. For example, when using Firefox I noticed that 100vw means the full width of viewport, extending under scrollbar (where content cannot expand!), so the fullwidth text has to be margined carefully and preferably get tested with many browsers and devices.

There is a big philosophy for this issue.
The easiest thing to do would be to give a certain font-size to body (I recommend 10), and then all the other element would have their font in em or rem.
I'll give you an example to understand those units.
Em is always relative to its parent:
body{font-size: 10px;}
.menu{font-size: 2em;} /* That means 2*10 pixels = 20 pixels */
.menu li{font-size: 1.5em;} /* That means 1.5*20 pixels = 30 pixels */
Rem is always relative to body:
body{font-size: 10px;}
.menu{font-size: 2rem;} /* That means 2*10 pixels = 20 pixels */
.menu li{font-size: 1.5rem;} /* that means 1.5*10 pixels = 15 pixels */
And then you could create a script that would modify font-size relative to your container width.
But this isn't what I would recommend. Because in a 900 pixels width container for example you would have a p element with a 12 pixels font-size let's say. And on your idea that would become an 300 pixels wide container at 4 pixels font-size. There has to be a lower limit.
Other solutions would be with media queries, so that you could set font for different widths.
But the solutions that I would recommend is to use a JavaScript library that helps you with that. And fittext.js that I found so far.

Here is the function:
document.body.setScaledFont = function(f) {
var s = this.offsetWidth, fs = s * f;
this.style.fontSize = fs + '%';
return this
};
Then convert all your documents child element font sizes to em's or %.
Then add something like this to your code to set the base font size.
document.body.setScaledFont(0.35);
window.onresize = function() {
document.body.setScaledFont(0.35);
}
http://jsfiddle.net/0tpvccjt/

There is a way to do this without JavaScript!
You can use an inline SVG image. You can use CSS on an SVG if it is inline. You have to remember that using this method means your SVG image will respond to its container size.
Try using the following solution...
HTML
<div>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 360.96 358.98" >
<text>SAVE $500</text>
</svg>
</div>
CSS
div {
width: 50%; /* Set your container width */
height: 50%; /* Set your container height */
}
svg {
width: 100%;
height: auto;
}
text {
transform: translate(40px, 202px);
font-size: 62px;
fill: #000;
}
Example:
https://jsfiddle.net/k8L4xLLa/32/
Want something more flashy?
SVG images also allow you to do cool stuff with shapes and junk. Check out this great use case for scalable text...
https://jsfiddle.net/k8L4xLLa/14/

CSS Container Queries
A late-2022 addition to the CSS feature set makes scaling font size with containers straightforward.
Container queries come with a new set of CSS units cqw/cqh (container query width/height). To use them you need to set the container-type property on the parent element whose size you want to use. Minimal example:
<div>
<p>Lorem ipsum dolor sit amet</p>
</div>
<style>
div {
container-type: inline-size;
}
p {
font-size: 5cqw;
}
</style>
The font size will increase smoothly as the container grows. At 1000px container width, the p font size will be 1000px / 100 * 5 = 50px.
container-type can be size or inline-size. size tracks both height and width of the container which allows you to use both cqw and cqh.
Most of the time on the web, heights are calculated based on content and you only specify the width. To save the browser some work, you'll generally want to set container-type: inline-size; so the browser only tracks the inline dimension which is usually width (unless you set writing-mode to vertical).
Browser support for container queries has grown rapidly in the 2nd half of 2022 and currently stands at 75% (2023-01-01).

This may not be super practical, but if you want a font to be a direct function of the parent, without having any JavaScript that listens/loops (interval) to read the size of the div/page, there is a way to do it. Iframes.
Anything within the iframe will consider the size of the iframe as the size of the viewport. So the trick is to just make an iframe whose width is the maximum width you want your text to be, and whose height is equal to the maximum height * the particular text's aspect ratio.
Setting aside the limitation that viewport units can't also come along side parent units for text (as in, having the % size behave like everyone else), viewport units do provide a very powerful tool: being able to get the minimum/maximum dimension. You can't do that anywhere else - you can't say...make the height of this div be the width of the parent * something.
That being said, the trick is to use vmin, and to set the iframe size so that [fraction] * total height is a good font size when the height is the limiting dimension, and [fraction] * total width when the width is the limiting dimension. This is why the height has to be a product of the width and the aspect ratio.
For my particular example, you have
.main iframe{
position: absolute;
top: 50%;
left: 50%;
width: 100%;
height: calc(3.5 * 100%);
background: rgba(0, 0, 0, 0);
border-style: none;
transform: translate3d(-50%, -50%, 0);
}
The small annoyance with this method is that you have to manually set the CSS of the iframe. If you attach the whole CSS file, that would take up a lot of bandwidth for many text areas. So, what I do is attach the rule that I want directly from my CSS.
var rule = document.styleSheets[1].rules[4];
var iDoc = document.querySelector('iframe').contentDocument;
iDoc.styleSheets[0].insertRule(rule.cssText);
You can write small function that gets the CSS rule / all CSS rules that would affect the text area.
I cannot think of another way to do it without having some cycling/listening JavaScript. The real solution would be for browsers to provide a way to scale text as a function of the parent container and to also provide the same vmin/vmax type functionality.
JSFiddle:
https://jsfiddle.net/0jr7rrgm/3/
(click once to lock the red square to the mouse, and click again to release)
Most of the JavaScript in the fiddle is just my custom click-drag function.

Using vw, em & co. works for sure, but IMO it always needs a human's touch for fine-tuning.
Here's a script I just wrote based on #tnt-rox' answer that tries to automatize that human's touch:
$('#controller').click(function(){
$('h2').each(function(){
var
$el = $(this),
max = $el.get(0),
el = null
;
max =
max
? max.offsetWidth
: 320
;
$el.css({
'font-size': '1em',
'display': 'inline',
});
el = $el.get(0);
el.get_float = function(){
var
fs = 0
;
if (this.style && this.style.fontSize) {
fs = parseFloat(this.style.fontSize.replace(/([\d\.]+)em/g, '$1'));
}
return fs;
};
el.bigger = function(){
this.style.fontSize = (this.get_float() + 0.1) + 'em';
};
while (el.offsetWidth < max) {
el.bigger();
}
// Finishing touch.
$el.css({
'font-size': ((el.get_float() -0.1) +'em'),
'line-height': 'normal',
'display': '',
});
}); // end of (each)
}); // end of (font scaling test)
div {
width: 50%;
background-color: tomato;
font-family: 'Arial';
}
h2 {
white-space: nowrap;
}
h2:nth-child(2) {
font-style: italic;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<input type="button" id="controller" value="Apply" />
<div>
<h2>Lorem ipsum dolor</h2>
<h2>Test String</h2>
<h2>Sweet Concatenation</h2>
<h2>Font Scaling</h2>
</div>
It basically reduces the font-size to 1em and then starts incrementing by 0.1 until it reaches maximum width.
JSFiddle

Use CSS Variables
No one has mentioned CSS variables yet, and this approach worked best for me, so:
Let's say you've got a column on your page that is 100% of the width of a mobile user's screen, but has a max-width of 800px, so on desktop there's some space on either side of the column. Put this at the top of your page:
<script> document.documentElement.style.setProperty('--column-width', Math.min(window.innerWidth, 800)+'px'); </script>
And now you can use that variable (instead of the built-in vw unit) to set the size of your font. E.g.
p {
font-size: calc( var(--column-width) / 100 );
}
It's not a pure CSS approach, but it's pretty close.

100% is relative to the base font size, which, if you haven't set it, would be the browser's user-agent default.
To get the effect you're after, I would use a piece of JavaScript code to adjust the base font size relative to the window dimensions.

Artistically, if you need to fit two or more lines of text within the same width regardless of their character count then you have nice options.
It's best to find a dynamical solution so whatever text is entered we end up with a nice display.
Let's see how we may approach.
var els = document.querySelectorAll(".divtext"),
refWidth = els[0].clientWidth,
refFontSize = parseFloat(window.getComputedStyle(els[0],null)
.getPropertyValue("font-size"));
els.forEach((el,i) => el.style.fontSize = refFontSize * refWidth / els[i].clientWidth + "px")
#container {
display: inline-block;
background-color: black;
padding: 0.6vw 1.2vw;
}
.divtext {
display: table;
color: white;
font-family: impact;
font-size: 4.5vw;
}
<div id="container">
<div class="divtext">THIS IS JUST AN</div>
<div class="divtext">EXAMPLE</div>
<div class="divtext">TO SHOW YOU WHAT</div>
<div class="divtext">YOU WANT</div>
</div>
All we do is to get the width (els[0].clientWidth) and the font size (parseFloat(window.getComputedStyle(els[0],null).getPropertyValue("font-size"))) of the first line as a reference and then just calculate the subsequent lines font size accordingly.

This web component changes the font size so the inner text width matches the container width. Check the demo.
You can use it like this:
<full-width-text>Lorem Ipsum</full-width-text>

You may be you looking for something like this:
http://jsfiddle.net/sijav/dGsC9/4/
http://fiddle.jshell.net/sijav/dGsC9/4/show/
I have used flowtype, and it's working great (however it's JavaScript and not a pure CSS solution):
$('body').flowtype({
minFont: 10,
maxFont: 40,
minimum: 500,
maximum: 1200,
fontRatio: 70
});

I've prepared a simple scale function using CSS transform instead of font-size. You can use it inside of any container, you don't have to set media queries, etc. :)
Blog post:
Full width CSS & JS scalable header
The code:
function scaleHeader() {
var scalable = document.querySelectorAll('.scale--js');
var margin = 10;
for (var i = 0; i < scalable.length; i++) {
var scalableContainer = scalable[i].parentNode;
scalable[i].style.transform = 'scale(1)';
var scalableContainerWidth = scalableContainer.offsetWidth - margin;
var scalableWidth = scalable[i].offsetWidth;
scalable[i].style.transform = 'scale(' + scalableContainerWidth / scalableWidth + ')';
scalableContainer.style.height = scalable[i].getBoundingClientRect().height + 'px';
}
}
Working demo:
https://codepen.io/maciejkorsan/pen/BWLryj

Inside your CSS, try adding this at the bottom changing the 320 pixels width for wherever your design starts breaking:
#media only screen and (max-width: 320px) {
body { font-size: 1em; }
}
Then give the font-size in "px" or "em" as you wish.

Try http://simplefocus.com/flowtype/. This is what I use for my sites, and it has worked perfectly.

My own solution, jQuery-based, works by gradually increasing the font size until the container gets a big increase in height (meaning it got a line break).
It's pretty simple, but works fairly well, and it is very easy to use. You don't have to know anything about the font being used, everything is taken care of by the browser.
You can play with it on http://jsfiddle.net/tubededentifrice/u5y15d0L/2/
The magic happens here:
var setMaxTextSize=function(jElement) {
// Get and set the font size into data for reuse upon resize
var fontSize=parseInt(jElement.data(quickFitFontSizeData)) || parseInt(jElement.css("font-size"));
jElement.data(quickFitFontSizeData, fontSize);
// Gradually increase font size until the element gets a big increase in height (i.e. line break)
var i = 0;
var previousHeight;
do
{
previousHeight=jElement.height();
jElement.css("font-size", "" + (++fontSize) + "px");
}
while(i++ < 300 && jElement.height()-previousHeight < fontSize/2)
// Finally, go back before the increase in height and set the element as resized by adding quickFitSetClass
fontSize -= 1;
jElement.addClass(quickFitSetClass).css("font-size", "" + fontSize + "px");
return fontSize;
};

My problem was similar, but related to scaling text within a heading. I tried Fit Font, but I needed to toggle the compressor to get any results, since it was solving a slightly different problem, as was Text Flow.
So I wrote my own little plugin that reduced the font size to fit the container, assuming you have overflow: hidden and white-space: nowrap so that even if reducing the font to the minimum doesn't allow showing the full heading, it just cuts off what it can show.
(function($) {
// Reduces the size of text in the element to fit the parent.
$.fn.reduceTextSize = function(options) {
options = $.extend({
minFontSize: 10
}, options);
function checkWidth(em) {
var $em = $(em);
var oldPosition = $em.css('position');
$em.css('position', 'absolute');
var width = $em.width();
$em.css('position', oldPosition);
return width;
}
return this.each(function(){
var $this = $(this);
var $parent = $this.parent();
var prevFontSize;
while (checkWidth($this) > $parent.width()) {
var currentFontSize = parseInt($this.css('font-size').replace('px', ''));
// Stop looping if min font size reached, or font size did not change last iteration.
if (isNaN(currentFontSize) || currentFontSize <= options.minFontSize ||
prevFontSize && prevFontSize == currentFontSize) {
break;
}
prevFontSize = currentFontSize;
$this.css('font-size', (currentFontSize - 1) + 'px');
}
});
};
})(jQuery);

Try to use the fitText plugin, because Viewport sizes isn't the solution of this problem.
Just add the library:
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
And change font-size for correct by settings the coefficient of text:
$("#text_div").fitText(0.8);
You can set maximum and minimum values of text:
$("#text_div").fitText(0.8, { minFontSize: '12px', maxFontSize: '36px' });

Here is a pure CSS solution with the understanding that you admit breakpoints are necessary but also want text scaling:
I'm aware I can add breakpoints, but I really want the text to scale
as well as having extra breakpoints, otherwise....
Here is an approach using:
Custom properties
Media queries for breakpoints
clamp() (browser support in Feb 2022 is pretty good at 93%)
calc()
If one common scaling factor can be used to control ALL the text scaling within a container per screen max-width, all you need to do is scale a custom property per max-width, and apply this factor to 1 calculation.
A basic setup starts like this:
:root {
--scaling-factor: 1
}
.parent {
font-size: 30px
}
.largest {
font-size: clamp(60%, calc(var(--scaling-factor) * 100%), 100%);
}
.middle {
font-size: clamp(60%, calc(var(--scaling-factor) * 85%), 100%);
}
.smallest {
font-size: clamp(60%, calc(var(--scaling-factor) * 70%), 100%);
}
Then nest your media queries something like this (or whatever you need for your breakpoints):
#media (max-width: 1200px) {
:root {
--scaling-factor: 0.9
}
#media (max-width: 800px) {
:root {
--scaling-factor: 0.8
}
#media (max-width: 600px) {
:root {
--scaling-factor: 0.5 /* nope, because the font-size is floored at 60% thanks to clamp() */
}
}
}
}
This minimizes your media query markup.
Advantages
One custom property controls ALL scaling ... no need to add multiple declarations per media breakpoint
The use of clamp() sets a lower-limit on what the font-size should be, so you ensure your text is never too small (here the floor is 60% of the parent's font-size)
Please see this JSFiddle for a demo. Resize the window until at the smallest widths, the paragraphs are all the same font-size.

Always have your element with this attribute:
JavaScript: element.style.fontSize = "100%";
or
CSS: style = "font-size: 100%;"
When you go fullscreen, you should already have a scale variable calculated (scale > 1 or scale = 1). Then, on fullscreen:
document.body.style.fontSize = (scale * 100) + "%";
It works nicely with little code.

Take look at my code. It makes the font size smaller to fit whatever there.
But I think this doesn't lead to a good user experience
var containerWidth = $("#ui-id-2").width();
var items = $(".quickSearchAutocomplete .ui-menu-item");
var fontSize = 16;
items.each(function(){
// Displaying a value depends sometimes on your case. You may make it block or inline-table instead of inline-block or whatever value that make the div take overflow width.
$(this).css({"whiteSpace": "nowrap", "display": "inline-block"});
while ($(this).width() > containerWidth){
console.log("$(this).width()" + $(this).width() + "containerWidth" + containerWidth)
$(this).css("font-size", fontSize -= 0.5);
}
});

For dynamic text, this plugin is quite useful:
http://freqdec.github.io/slabText/
Simply add CSS:
.slabtexted .slabtext
{
display: -moz-inline-box;
display: inline-block;
white-space: nowrap;
}
.slabtextinactive .slabtext
{
display: inline;
white-space: normal;
font-size: 1em !important;
letter-spacing: inherit !important;
word-spacing: inherit !important;
*letter-spacing: normal !important;
*word-spacing: normal !important;
}
.slabtextdone .slabtext
{
display: block;
}
And the script:
$('#mydiv').slabText();

This worked for me:
I try to approximate font-size based on a width/height got from setting `font-size: 10px`. Basically, the idea is "if I have 20 pixels width and 11 pixels height with `font-size: 10px`, so what would it be the maximum font-size to math a container of 50 pixels width and 30 pixels height?"
The answer is a double proportion system:
{ 20:10=50:X, 11:10=30:Y } = { X= (10*50)/20, Y= (10*30)/11 }
Now X is a font-size that will match width, and Y is a font-size that will match height; take the smallest value
function getMaxFontSizeApprox(el){
var fontSize = 10;
var p = el.parentNode;
var parent_h = p.offsetHeight ? p.offsetHeight : p.style.pixelHeight;
if(!parent_h)
parent_h = 0;
var parent_w = p.offsetHeight ? p.offsetWidth : p.style.pixelWidth;
if(!parent_w)
parent_w = 0;
el.style.fontSize = fontSize + "px";
var el_h = el.offsetHeight ? el.offsetHeight : el.style.pixelHeight;
if(!el_h)
el_h = 0;
var el_w = el.offsetHeight ? el.offsetWidth : el.style.pixelWidth;
if(!el_w)
el_w = 0;
// 0.5 is the error on the measure that JavaScript does
// if the real measure had been 12.49 px => JavaScript would have said 12px
// so we think about the worst case when could have, we add 0.5 to
// compensate the round error
var fs1 = (fontSize*(parent_w + 0.5))/(el_w + 0.5);
var fs2 = (fontSize*(parent_h) + 0.5)/(el_h + 0.5);
fontSize = Math.floor(Math.min(fs1,fs2));
el.style.fontSize = fontSize + "px";
return fontSize;
}
NB: the argument of the function must be a span element or an element which is smaller than its parent, otherwise if children and parent have both the same width/height function will fail.

let textElement = document.getElementById('text1');
let parentElement = textElement.parentElement;
const parentClientHeight = parentElement.clientHeight;
const parentClientWidth = parentElement.clientWidth;
textElement.style.padding = "unset";
textElement.style.margin = "auto";
let fontSize = parentClientHeight;
let minFS = 3,
maxFS = fontSize;
while (fontSize != minFS) {
textElement.style.fontSize = `${fontSize}px`;
if (
parentElement.scrollHeight <= parentClientHeight &&
parentElement.scrollWidth <= parentClientWidth
) {
minFS = fontSize;
} else {
maxFS = fontSize;
}
fontSize = Math.floor((minFS + maxFS) / 2);
}
textElement.style.fontSize = `${minFS}px`;
<div style="height:200px; width:300px;">
<div id='text1'>
test
</div>
</div>

As a JavaScript fallback (or your sole solution), you can use my jQuery Scalem plugin, which lets you scale relative to the parent element (container) by passing the reference option.

In case it's helpful to anyone, most of the solutions in this thread were wrapping text into multiple lines, form e.
But then I found this, and it worked:
https://github.com/chunksnbits/jquery-quickfit
Example usage:
$('.someText').quickfit({max:50,tolerance:.4})

I'm having a hard time getting my head around font scaling.
I currently have a website with a body font-size of 100%. 100% of what though? This seems to compute out at 16 pixels.
I was under the impression that 100% would somehow refer to the size of the browser window, but apparently not because it's always 16 pixels whether the window is resized down to a mobile width or full-blown widescreen desktop.
How can I make the text on my site scale in relation to its container? I tried using em, but this doesn't scale either.
My reasoning is that things like my menu become squished when you resize, so I need to reduce the px font-size of .menuItem among other elements in relation to the width of the container. (For example, in the menu on a large desktop, 22px works perfectly. Move down to tablet width and 16px is more appropriate.)
I'm aware I can add breakpoints, but I really want the text to scale as well as having extra breakpoints, otherwise, I'll end up with hundreds of breakpoints for every 100pixels decrease in width to control the text.

If the container is not the body, CSS Tricks covers all of your options in Fitting Text to a Container.
If the container is the body, what you are looking for is Viewport-percentage lengths:
The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly. However, when the value of overflow on the root element is auto, any scroll bars are assumed not to exist.
The values are:
vw (% of the viewport width)
vh (% of the viewport height)
vi (1% of the viewport size in the direction of the root element's inline axis)
vb (1% of the viewport size in the direction of the root element's block axis)
vmin (the smaller of vw or vh)
vmax (the larger or vw or vh)
1 v* is equal to 1% of the initial containing block.
Using it looks like this:
p {
font-size: 4vw;
}
As you can see, when the viewport width increases, so do the font-size, without needing to use media queries.
These values are a sizing unit, just like px or em, so they can be used to size other elements as well, such as width, margin, or padding.
Browser support is pretty good, but you'll likely need a fallback, such as:
p {
font-size: 16px;
font-size: 4vw;
}
Check out the support statistics: http://caniuse.com/#feat=viewport-units.
Also, check out CSS-Tricks for a broader look: Viewport Sized Typography
Here's a nice article about setting minimum/maximum sizes and exercising a bit more control over the sizes: Precise control over responsive typography
And here's an article about setting your size using calc() so that the text fills the viewport: http://codepen.io/CrocoDillon/pen/fBJxu
Also, please view this article, which uses a technique dubbed 'molten leading' to adjust the line-height as well. Molten Leading in CSS

But what if the container is not the viewport (body)?
This question is asked in a comment by Alex under 2507rkt3's answer.
That fact does not mean vw cannot be used to some extent to size for that container. Now to see any variation at all one has to be assuming that the container in some way is flexible in size. Whether through a direct percentage width or through being 100% minus margins. The point becomes "moot" if the container is always set to, let's say, 200px wide--then just set a font-size that works for that width.
Example 1
With a flexible width container, however, it must be realized that in some way the container is still being sized off the viewport. As such, it is a matter of adjusting a vw setting based off that percentage size difference to the viewport, which means taking into account the sizing of parent wrappers. Take this example:
div {
width: 50%;
border: 1px solid black;
margin: 20px;
font-size: 16px;
/* 100 = viewport width, as 1vw = 1/100th of that
So if the container is 50% of viewport (as here)
then factor that into how you want it to size.
Let's say you like 5vw if it were the whole width,
then for this container, size it at 2.5vw (5 * .5 [i.e. 50%])
*/
font-size: 2.5vw;
}
Assuming here the div is a child of the body, it is 50% of that 100% width, which is the viewport size in this basic case. Basically, you want to set a vw that is going to look good to you. As you can see in my comment in the above CSS content, you can "think" through that mathematically with respect to the full viewport size, but you don't need to do that. The text is going to "flex" with the container because the container is flexing with the viewport resizing. Here's an example of two differently sized containers.
Example 2
You can help ensure viewport sizing by forcing the calculation based off that. Consider this example:
html {width: 100%;} /* Force 'html' to be viewport width */
body {width: 150%; } /* Overflow the body */
div {
width: 50%;
border: 1px solid black;
margin: 20px;
font-size: 16px;
/* 100 = viewport width, as 1vw = 1/100th of that
Here, the body is 150% of viewport, but the container is 50%
of viewport, so both parents factor into how you want it to size.
Let's say you like 5vw if it were the whole width,
then for this container, size it at 3.75vw
(5 * 1.5 [i.e. 150%]) * .5 [i.e. 50%]
*/
font-size: 3.75vw;
}
The sizing is still based off viewport, but is in essence set up based off the container size itself.
Should the Size of the Container Change Dynamically...
If the sizing of the container element ended up changing dynamically its percentage relationship either via #media breakpoints or via JavaScript, then whatever the base "target" was would need recalculation to maintain the same "relationship" for text sizing.
Take example #1 above. If the div was switched to 25% width by either #media or JavaScript, then at the same time, the font-size would need to adjust in either the media query or by JavaScript to the new calculation of 5vw * .25 = 1.25. This would put the text size at the same size it would have been had the "width" of the original 50% container been reduced by half from viewport sizing, but has now been reduced due to a change in its own percentage calculation.
A Challenge
With the CSS calc() function in use, it would become difficult to adjust dynamically, as that function does not work for font-size purposes at this time. So you could not do a pure CSS adjustment if your width is changing on calc(). Of course, a minor adjustment of width for margins may not be enough to warrant any change in font-size, so it may not matter.

Solution with SVG:
.resizeme {
resize: both;
margin: 0;
padding: 0;
height: 75px;
width: 500px;
background-color: lightblue;
overflow: hidden;
}
<div class="resizeme">
<svg
width="100%"
height="100%"
viewBox="0 0 500 75"
preserveAspectRatio="xMinYMid meet"
style="background-color:green"
xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink"
>
<text
x="0"
y="75"
font-size="75"
fill="black"
>█Resize This█</text>
</svg>
</div>
Solution with SVG and text-wrapping using foreignObject:
.resizeme {
resize: both;
margin: 0;
padding: 0;
height: 200px;
width: 500px;
background-color: lightblue;
overflow: hidden;
}
<div class="resizeme">
<svg
width="100%"
height="100%"
viewBox="0 0 500 200"
preserveAspectRatio="xMinYMin meet"
>
<foreignObject width="100%" height="100%" xmlns="http://www.w3.org/1999/xhtml">
<div xmlns="http://www.w3.org/1999/xhtml" style="background-color:lightgreen;">
<h1>heading</h1>
<p>Resize the blue box.</p>
</div>
</foreignObject>
</svg>
</div>

In one of my projects I use a "mixture" between vw and vh to adjust the font size to my needs, for example:
font-size: calc(3vw + 3vh);
I know this doesn't answer the OP's question, but maybe it can be a solution to anyone else.

Pure-CSS solution with calc(), CSS units and math
This is precisely not what OP asks, but may make someone's day. This answer is not spoon-feedingly easy and needs some researching on the developer end.
I came finally to get a pure-CSS solution for this using calc() with different units. You will need some basic mathematical understanding of formulas to work out your expression for calc().
When I worked this out, I had to get a full-page-width responsive header with some padding few parents up in DOM. I'll use my values here, replace them with your own.
To mathematics
You will need:
Nicely adjusted ratio in some viewport. I used 320 pixels, thus I got 24 pixels high and 224 pixels wide, so the ratio is 9.333... or 28 / 3
The container width, I had padding: 3em and full width so this got to 100wv - 2 * 3em
X is the width of container, so replace it with your own expression or adjust the value to get full-page text. R is the ratio you will have. You can get it by adjusting the values in some viewport, inspecting element width and height and replacing them with your own values. Also, it is width / heigth ;)
x = 100vw - 2 * 3em = 100vw - 6em
r = 224px/24px = 9.333... = 28 / 3
y = x / r
= (100vw - 6em) / (28 / 3)
= (100vw - 6em) * 3 / 28
= (300vw - 18em) / 28
= (75vw - 4.5rem) / 7
And bang! It worked! I wrote
font-size: calc((75vw - 4.5rem) / 7)
to my header and it adjusted nicely in every viewport.
But how does it work?
We need some constants up here. 100vw means the full width of viewport, and my goal was to establish full-width header with some padding.
The ratio. Getting a width and height in one viewport got me a ratio to play with, and with ratio I know what the height should be in other viewport width. Calculating them with hand would take plenty of time and at least take lots of bandwidth, so it's not a good answer.
Conclusion
I wonder why no-one has figured this out and some people are even telling that this would be impossible to tinker with CSS. I don't like to use JavaScript in adjusting elements, so I don't accept JavaScript (and forget about jQuery) answers without digging more. All in all, it's good that this got figured out and this is one step to pure-CSS implementations in website design.
I apologize of any unusual convention in my text, I'm not native speaker in English and am also quite new to writing Stack Overflow answers.
It should also be noted that we have evil scrollbars in some browsers. For example, when using Firefox I noticed that 100vw means the full width of viewport, extending under scrollbar (where content cannot expand!), so the fullwidth text has to be margined carefully and preferably get tested with many browsers and devices.

There is a big philosophy for this issue.
The easiest thing to do would be to give a certain font-size to body (I recommend 10), and then all the other element would have their font in em or rem.
I'll give you an example to understand those units.
Em is always relative to its parent:
body{font-size: 10px;}
.menu{font-size: 2em;} /* That means 2*10 pixels = 20 pixels */
.menu li{font-size: 1.5em;} /* That means 1.5*20 pixels = 30 pixels */
Rem is always relative to body:
body{font-size: 10px;}
.menu{font-size: 2rem;} /* That means 2*10 pixels = 20 pixels */
.menu li{font-size: 1.5rem;} /* that means 1.5*10 pixels = 15 pixels */
And then you could create a script that would modify font-size relative to your container width.
But this isn't what I would recommend. Because in a 900 pixels width container for example you would have a p element with a 12 pixels font-size let's say. And on your idea that would become an 300 pixels wide container at 4 pixels font-size. There has to be a lower limit.
Other solutions would be with media queries, so that you could set font for different widths.
But the solutions that I would recommend is to use a JavaScript library that helps you with that. And fittext.js that I found so far.

Here is the function:
document.body.setScaledFont = function(f) {
var s = this.offsetWidth, fs = s * f;
this.style.fontSize = fs + '%';
return this
};
Then convert all your documents child element font sizes to em's or %.
Then add something like this to your code to set the base font size.
document.body.setScaledFont(0.35);
window.onresize = function() {
document.body.setScaledFont(0.35);
}
http://jsfiddle.net/0tpvccjt/

There is a way to do this without JavaScript!
You can use an inline SVG image. You can use CSS on an SVG if it is inline. You have to remember that using this method means your SVG image will respond to its container size.
Try using the following solution...
HTML
<div>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 360.96 358.98" >
<text>SAVE $500</text>
</svg>
</div>
CSS
div {
width: 50%; /* Set your container width */
height: 50%; /* Set your container height */
}
svg {
width: 100%;
height: auto;
}
text {
transform: translate(40px, 202px);
font-size: 62px;
fill: #000;
}
Example:
https://jsfiddle.net/k8L4xLLa/32/
Want something more flashy?
SVG images also allow you to do cool stuff with shapes and junk. Check out this great use case for scalable text...
https://jsfiddle.net/k8L4xLLa/14/

CSS Container Queries
A late-2022 addition to the CSS feature set makes scaling font size with containers straightforward.
Container queries come with a new set of CSS units cqw/cqh (container query width/height). To use them you need to set the container-type property on the parent element whose size you want to use. Minimal example:
<div>
<p>Lorem ipsum dolor sit amet</p>
</div>
<style>
div {
container-type: inline-size;
}
p {
font-size: 5cqw;
}
</style>
The font size will increase smoothly as the container grows. At 1000px container width, the p font size will be 1000px / 100 * 5 = 50px.
container-type can be size or inline-size. size tracks both height and width of the container which allows you to use both cqw and cqh.
Most of the time on the web, heights are calculated based on content and you only specify the width. To save the browser some work, you'll generally want to set container-type: inline-size; so the browser only tracks the inline dimension which is usually width (unless you set writing-mode to vertical).
Browser support for container queries has grown rapidly in the 2nd half of 2022 and currently stands at 75% (2023-01-01).

This may not be super practical, but if you want a font to be a direct function of the parent, without having any JavaScript that listens/loops (interval) to read the size of the div/page, there is a way to do it. Iframes.
Anything within the iframe will consider the size of the iframe as the size of the viewport. So the trick is to just make an iframe whose width is the maximum width you want your text to be, and whose height is equal to the maximum height * the particular text's aspect ratio.
Setting aside the limitation that viewport units can't also come along side parent units for text (as in, having the % size behave like everyone else), viewport units do provide a very powerful tool: being able to get the minimum/maximum dimension. You can't do that anywhere else - you can't say...make the height of this div be the width of the parent * something.
That being said, the trick is to use vmin, and to set the iframe size so that [fraction] * total height is a good font size when the height is the limiting dimension, and [fraction] * total width when the width is the limiting dimension. This is why the height has to be a product of the width and the aspect ratio.
For my particular example, you have
.main iframe{
position: absolute;
top: 50%;
left: 50%;
width: 100%;
height: calc(3.5 * 100%);
background: rgba(0, 0, 0, 0);
border-style: none;
transform: translate3d(-50%, -50%, 0);
}
The small annoyance with this method is that you have to manually set the CSS of the iframe. If you attach the whole CSS file, that would take up a lot of bandwidth for many text areas. So, what I do is attach the rule that I want directly from my CSS.
var rule = document.styleSheets[1].rules[4];
var iDoc = document.querySelector('iframe').contentDocument;
iDoc.styleSheets[0].insertRule(rule.cssText);
You can write small function that gets the CSS rule / all CSS rules that would affect the text area.
I cannot think of another way to do it without having some cycling/listening JavaScript. The real solution would be for browsers to provide a way to scale text as a function of the parent container and to also provide the same vmin/vmax type functionality.
JSFiddle:
https://jsfiddle.net/0jr7rrgm/3/
(click once to lock the red square to the mouse, and click again to release)
Most of the JavaScript in the fiddle is just my custom click-drag function.

Using vw, em & co. works for sure, but IMO it always needs a human's touch for fine-tuning.
Here's a script I just wrote based on #tnt-rox' answer that tries to automatize that human's touch:
$('#controller').click(function(){
$('h2').each(function(){
var
$el = $(this),
max = $el.get(0),
el = null
;
max =
max
? max.offsetWidth
: 320
;
$el.css({
'font-size': '1em',
'display': 'inline',
});
el = $el.get(0);
el.get_float = function(){
var
fs = 0
;
if (this.style && this.style.fontSize) {
fs = parseFloat(this.style.fontSize.replace(/([\d\.]+)em/g, '$1'));
}
return fs;
};
el.bigger = function(){
this.style.fontSize = (this.get_float() + 0.1) + 'em';
};
while (el.offsetWidth < max) {
el.bigger();
}
// Finishing touch.
$el.css({
'font-size': ((el.get_float() -0.1) +'em'),
'line-height': 'normal',
'display': '',
});
}); // end of (each)
}); // end of (font scaling test)
div {
width: 50%;
background-color: tomato;
font-family: 'Arial';
}
h2 {
white-space: nowrap;
}
h2:nth-child(2) {
font-style: italic;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<input type="button" id="controller" value="Apply" />
<div>
<h2>Lorem ipsum dolor</h2>
<h2>Test String</h2>
<h2>Sweet Concatenation</h2>
<h2>Font Scaling</h2>
</div>
It basically reduces the font-size to 1em and then starts incrementing by 0.1 until it reaches maximum width.
JSFiddle

Use CSS Variables
No one has mentioned CSS variables yet, and this approach worked best for me, so:
Let's say you've got a column on your page that is 100% of the width of a mobile user's screen, but has a max-width of 800px, so on desktop there's some space on either side of the column. Put this at the top of your page:
<script> document.documentElement.style.setProperty('--column-width', Math.min(window.innerWidth, 800)+'px'); </script>
And now you can use that variable (instead of the built-in vw unit) to set the size of your font. E.g.
p {
font-size: calc( var(--column-width) / 100 );
}
It's not a pure CSS approach, but it's pretty close.

100% is relative to the base font size, which, if you haven't set it, would be the browser's user-agent default.
To get the effect you're after, I would use a piece of JavaScript code to adjust the base font size relative to the window dimensions.

Artistically, if you need to fit two or more lines of text within the same width regardless of their character count then you have nice options.
It's best to find a dynamical solution so whatever text is entered we end up with a nice display.
Let's see how we may approach.
var els = document.querySelectorAll(".divtext"),
refWidth = els[0].clientWidth,
refFontSize = parseFloat(window.getComputedStyle(els[0],null)
.getPropertyValue("font-size"));
els.forEach((el,i) => el.style.fontSize = refFontSize * refWidth / els[i].clientWidth + "px")
#container {
display: inline-block;
background-color: black;
padding: 0.6vw 1.2vw;
}
.divtext {
display: table;
color: white;
font-family: impact;
font-size: 4.5vw;
}
<div id="container">
<div class="divtext">THIS IS JUST AN</div>
<div class="divtext">EXAMPLE</div>
<div class="divtext">TO SHOW YOU WHAT</div>
<div class="divtext">YOU WANT</div>
</div>
All we do is to get the width (els[0].clientWidth) and the font size (parseFloat(window.getComputedStyle(els[0],null).getPropertyValue("font-size"))) of the first line as a reference and then just calculate the subsequent lines font size accordingly.

This web component changes the font size so the inner text width matches the container width. Check the demo.
You can use it like this:
<full-width-text>Lorem Ipsum</full-width-text>

You may be you looking for something like this:
http://jsfiddle.net/sijav/dGsC9/4/
http://fiddle.jshell.net/sijav/dGsC9/4/show/
I have used flowtype, and it's working great (however it's JavaScript and not a pure CSS solution):
$('body').flowtype({
minFont: 10,
maxFont: 40,
minimum: 500,
maximum: 1200,
fontRatio: 70
});

I've prepared a simple scale function using CSS transform instead of font-size. You can use it inside of any container, you don't have to set media queries, etc. :)
Blog post:
Full width CSS & JS scalable header
The code:
function scaleHeader() {
var scalable = document.querySelectorAll('.scale--js');
var margin = 10;
for (var i = 0; i < scalable.length; i++) {
var scalableContainer = scalable[i].parentNode;
scalable[i].style.transform = 'scale(1)';
var scalableContainerWidth = scalableContainer.offsetWidth - margin;
var scalableWidth = scalable[i].offsetWidth;
scalable[i].style.transform = 'scale(' + scalableContainerWidth / scalableWidth + ')';
scalableContainer.style.height = scalable[i].getBoundingClientRect().height + 'px';
}
}
Working demo:
https://codepen.io/maciejkorsan/pen/BWLryj

Inside your CSS, try adding this at the bottom changing the 320 pixels width for wherever your design starts breaking:
#media only screen and (max-width: 320px) {
body { font-size: 1em; }
}
Then give the font-size in "px" or "em" as you wish.

Try http://simplefocus.com/flowtype/. This is what I use for my sites, and it has worked perfectly.

My own solution, jQuery-based, works by gradually increasing the font size until the container gets a big increase in height (meaning it got a line break).
It's pretty simple, but works fairly well, and it is very easy to use. You don't have to know anything about the font being used, everything is taken care of by the browser.
You can play with it on http://jsfiddle.net/tubededentifrice/u5y15d0L/2/
The magic happens here:
var setMaxTextSize=function(jElement) {
// Get and set the font size into data for reuse upon resize
var fontSize=parseInt(jElement.data(quickFitFontSizeData)) || parseInt(jElement.css("font-size"));
jElement.data(quickFitFontSizeData, fontSize);
// Gradually increase font size until the element gets a big increase in height (i.e. line break)
var i = 0;
var previousHeight;
do
{
previousHeight=jElement.height();
jElement.css("font-size", "" + (++fontSize) + "px");
}
while(i++ < 300 && jElement.height()-previousHeight < fontSize/2)
// Finally, go back before the increase in height and set the element as resized by adding quickFitSetClass
fontSize -= 1;
jElement.addClass(quickFitSetClass).css("font-size", "" + fontSize + "px");
return fontSize;
};

My problem was similar, but related to scaling text within a heading. I tried Fit Font, but I needed to toggle the compressor to get any results, since it was solving a slightly different problem, as was Text Flow.
So I wrote my own little plugin that reduced the font size to fit the container, assuming you have overflow: hidden and white-space: nowrap so that even if reducing the font to the minimum doesn't allow showing the full heading, it just cuts off what it can show.
(function($) {
// Reduces the size of text in the element to fit the parent.
$.fn.reduceTextSize = function(options) {
options = $.extend({
minFontSize: 10
}, options);
function checkWidth(em) {
var $em = $(em);
var oldPosition = $em.css('position');
$em.css('position', 'absolute');
var width = $em.width();
$em.css('position', oldPosition);
return width;
}
return this.each(function(){
var $this = $(this);
var $parent = $this.parent();
var prevFontSize;
while (checkWidth($this) > $parent.width()) {
var currentFontSize = parseInt($this.css('font-size').replace('px', ''));
// Stop looping if min font size reached, or font size did not change last iteration.
if (isNaN(currentFontSize) || currentFontSize <= options.minFontSize ||
prevFontSize && prevFontSize == currentFontSize) {
break;
}
prevFontSize = currentFontSize;
$this.css('font-size', (currentFontSize - 1) + 'px');
}
});
};
})(jQuery);

Try to use the fitText plugin, because Viewport sizes isn't the solution of this problem.
Just add the library:
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
And change font-size for correct by settings the coefficient of text:
$("#text_div").fitText(0.8);
You can set maximum and minimum values of text:
$("#text_div").fitText(0.8, { minFontSize: '12px', maxFontSize: '36px' });

Here is a pure CSS solution with the understanding that you admit breakpoints are necessary but also want text scaling:
I'm aware I can add breakpoints, but I really want the text to scale
as well as having extra breakpoints, otherwise....
Here is an approach using:
Custom properties
Media queries for breakpoints
clamp() (browser support in Feb 2022 is pretty good at 93%)
calc()
If one common scaling factor can be used to control ALL the text scaling within a container per screen max-width, all you need to do is scale a custom property per max-width, and apply this factor to 1 calculation.
A basic setup starts like this:
:root {
--scaling-factor: 1
}
.parent {
font-size: 30px
}
.largest {
font-size: clamp(60%, calc(var(--scaling-factor) * 100%), 100%);
}
.middle {
font-size: clamp(60%, calc(var(--scaling-factor) * 85%), 100%);
}
.smallest {
font-size: clamp(60%, calc(var(--scaling-factor) * 70%), 100%);
}
Then nest your media queries something like this (or whatever you need for your breakpoints):
#media (max-width: 1200px) {
:root {
--scaling-factor: 0.9
}
#media (max-width: 800px) {
:root {
--scaling-factor: 0.8
}
#media (max-width: 600px) {
:root {
--scaling-factor: 0.5 /* nope, because the font-size is floored at 60% thanks to clamp() */
}
}
}
}
This minimizes your media query markup.
Advantages
One custom property controls ALL scaling ... no need to add multiple declarations per media breakpoint
The use of clamp() sets a lower-limit on what the font-size should be, so you ensure your text is never too small (here the floor is 60% of the parent's font-size)
Please see this JSFiddle for a demo. Resize the window until at the smallest widths, the paragraphs are all the same font-size.

Always have your element with this attribute:
JavaScript: element.style.fontSize = "100%";
or
CSS: style = "font-size: 100%;"
When you go fullscreen, you should already have a scale variable calculated (scale > 1 or scale = 1). Then, on fullscreen:
document.body.style.fontSize = (scale * 100) + "%";
It works nicely with little code.

Take look at my code. It makes the font size smaller to fit whatever there.
But I think this doesn't lead to a good user experience
var containerWidth = $("#ui-id-2").width();
var items = $(".quickSearchAutocomplete .ui-menu-item");
var fontSize = 16;
items.each(function(){
// Displaying a value depends sometimes on your case. You may make it block or inline-table instead of inline-block or whatever value that make the div take overflow width.
$(this).css({"whiteSpace": "nowrap", "display": "inline-block"});
while ($(this).width() > containerWidth){
console.log("$(this).width()" + $(this).width() + "containerWidth" + containerWidth)
$(this).css("font-size", fontSize -= 0.5);
}
});

For dynamic text, this plugin is quite useful:
http://freqdec.github.io/slabText/
Simply add CSS:
.slabtexted .slabtext
{
display: -moz-inline-box;
display: inline-block;
white-space: nowrap;
}
.slabtextinactive .slabtext
{
display: inline;
white-space: normal;
font-size: 1em !important;
letter-spacing: inherit !important;
word-spacing: inherit !important;
*letter-spacing: normal !important;
*word-spacing: normal !important;
}
.slabtextdone .slabtext
{
display: block;
}
And the script:
$('#mydiv').slabText();

This worked for me:
I try to approximate font-size based on a width/height got from setting `font-size: 10px`. Basically, the idea is "if I have 20 pixels width and 11 pixels height with `font-size: 10px`, so what would it be the maximum font-size to math a container of 50 pixels width and 30 pixels height?"
The answer is a double proportion system:
{ 20:10=50:X, 11:10=30:Y } = { X= (10*50)/20, Y= (10*30)/11 }
Now X is a font-size that will match width, and Y is a font-size that will match height; take the smallest value
function getMaxFontSizeApprox(el){
var fontSize = 10;
var p = el.parentNode;
var parent_h = p.offsetHeight ? p.offsetHeight : p.style.pixelHeight;
if(!parent_h)
parent_h = 0;
var parent_w = p.offsetHeight ? p.offsetWidth : p.style.pixelWidth;
if(!parent_w)
parent_w = 0;
el.style.fontSize = fontSize + "px";
var el_h = el.offsetHeight ? el.offsetHeight : el.style.pixelHeight;
if(!el_h)
el_h = 0;
var el_w = el.offsetHeight ? el.offsetWidth : el.style.pixelWidth;
if(!el_w)
el_w = 0;
// 0.5 is the error on the measure that JavaScript does
// if the real measure had been 12.49 px => JavaScript would have said 12px
// so we think about the worst case when could have, we add 0.5 to
// compensate the round error
var fs1 = (fontSize*(parent_w + 0.5))/(el_w + 0.5);
var fs2 = (fontSize*(parent_h) + 0.5)/(el_h + 0.5);
fontSize = Math.floor(Math.min(fs1,fs2));
el.style.fontSize = fontSize + "px";
return fontSize;
}
NB: the argument of the function must be a span element or an element which is smaller than its parent, otherwise if children and parent have both the same width/height function will fail.

let textElement = document.getElementById('text1');
let parentElement = textElement.parentElement;
const parentClientHeight = parentElement.clientHeight;
const parentClientWidth = parentElement.clientWidth;
textElement.style.padding = "unset";
textElement.style.margin = "auto";
let fontSize = parentClientHeight;
let minFS = 3,
maxFS = fontSize;
while (fontSize != minFS) {
textElement.style.fontSize = `${fontSize}px`;
if (
parentElement.scrollHeight <= parentClientHeight &&
parentElement.scrollWidth <= parentClientWidth
) {
minFS = fontSize;
} else {
maxFS = fontSize;
}
fontSize = Math.floor((minFS + maxFS) / 2);
}
textElement.style.fontSize = `${minFS}px`;
<div style="height:200px; width:300px;">
<div id='text1'>
test
</div>
</div>

As a JavaScript fallback (or your sole solution), you can use my jQuery Scalem plugin, which lets you scale relative to the parent element (container) by passing the reference option.

In case it's helpful to anyone, most of the solutions in this thread were wrapping text into multiple lines, form e.
But then I found this, and it worked:
https://github.com/chunksnbits/jquery-quickfit
Example usage:
$('.someText').quickfit({max:50,tolerance:.4})

I'm having a hard time getting my head around font scaling.
I currently have a website with a body font-size of 100%. 100% of what though? This seems to compute out at 16 pixels.
I was under the impression that 100% would somehow refer to the size of the browser window, but apparently not because it's always 16 pixels whether the window is resized down to a mobile width or full-blown widescreen desktop.
How can I make the text on my site scale in relation to its container? I tried using em, but this doesn't scale either.
My reasoning is that things like my menu become squished when you resize, so I need to reduce the px font-size of .menuItem among other elements in relation to the width of the container. (For example, in the menu on a large desktop, 22px works perfectly. Move down to tablet width and 16px is more appropriate.)
I'm aware I can add breakpoints, but I really want the text to scale as well as having extra breakpoints, otherwise, I'll end up with hundreds of breakpoints for every 100pixels decrease in width to control the text.

If the container is not the body, CSS Tricks covers all of your options in Fitting Text to a Container.
If the container is the body, what you are looking for is Viewport-percentage lengths:
The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly. However, when the value of overflow on the root element is auto, any scroll bars are assumed not to exist.
The values are:
vw (% of the viewport width)
vh (% of the viewport height)
vi (1% of the viewport size in the direction of the root element's inline axis)
vb (1% of the viewport size in the direction of the root element's block axis)
vmin (the smaller of vw or vh)
vmax (the larger or vw or vh)
1 v* is equal to 1% of the initial containing block.
Using it looks like this:
p {
font-size: 4vw;
}
As you can see, when the viewport width increases, so do the font-size, without needing to use media queries.
These values are a sizing unit, just like px or em, so they can be used to size other elements as well, such as width, margin, or padding.
Browser support is pretty good, but you'll likely need a fallback, such as:
p {
font-size: 16px;
font-size: 4vw;
}
Check out the support statistics: http://caniuse.com/#feat=viewport-units.
Also, check out CSS-Tricks for a broader look: Viewport Sized Typography
Here's a nice article about setting minimum/maximum sizes and exercising a bit more control over the sizes: Precise control over responsive typography
And here's an article about setting your size using calc() so that the text fills the viewport: http://codepen.io/CrocoDillon/pen/fBJxu
Also, please view this article, which uses a technique dubbed 'molten leading' to adjust the line-height as well. Molten Leading in CSS

But what if the container is not the viewport (body)?
This question is asked in a comment by Alex under 2507rkt3's answer.
That fact does not mean vw cannot be used to some extent to size for that container. Now to see any variation at all one has to be assuming that the container in some way is flexible in size. Whether through a direct percentage width or through being 100% minus margins. The point becomes "moot" if the container is always set to, let's say, 200px wide--then just set a font-size that works for that width.
Example 1
With a flexible width container, however, it must be realized that in some way the container is still being sized off the viewport. As such, it is a matter of adjusting a vw setting based off that percentage size difference to the viewport, which means taking into account the sizing of parent wrappers. Take this example:
div {
width: 50%;
border: 1px solid black;
margin: 20px;
font-size: 16px;
/* 100 = viewport width, as 1vw = 1/100th of that
So if the container is 50% of viewport (as here)
then factor that into how you want it to size.
Let's say you like 5vw if it were the whole width,
then for this container, size it at 2.5vw (5 * .5 [i.e. 50%])
*/
font-size: 2.5vw;
}
Assuming here the div is a child of the body, it is 50% of that 100% width, which is the viewport size in this basic case. Basically, you want to set a vw that is going to look good to you. As you can see in my comment in the above CSS content, you can "think" through that mathematically with respect to the full viewport size, but you don't need to do that. The text is going to "flex" with the container because the container is flexing with the viewport resizing. Here's an example of two differently sized containers.
Example 2
You can help ensure viewport sizing by forcing the calculation based off that. Consider this example:
html {width: 100%;} /* Force 'html' to be viewport width */
body {width: 150%; } /* Overflow the body */
div {
width: 50%;
border: 1px solid black;
margin: 20px;
font-size: 16px;
/* 100 = viewport width, as 1vw = 1/100th of that
Here, the body is 150% of viewport, but the container is 50%
of viewport, so both parents factor into how you want it to size.
Let's say you like 5vw if it were the whole width,
then for this container, size it at 3.75vw
(5 * 1.5 [i.e. 150%]) * .5 [i.e. 50%]
*/
font-size: 3.75vw;
}
The sizing is still based off viewport, but is in essence set up based off the container size itself.
Should the Size of the Container Change Dynamically...
If the sizing of the container element ended up changing dynamically its percentage relationship either via #media breakpoints or via JavaScript, then whatever the base "target" was would need recalculation to maintain the same "relationship" for text sizing.
Take example #1 above. If the div was switched to 25% width by either #media or JavaScript, then at the same time, the font-size would need to adjust in either the media query or by JavaScript to the new calculation of 5vw * .25 = 1.25. This would put the text size at the same size it would have been had the "width" of the original 50% container been reduced by half from viewport sizing, but has now been reduced due to a change in its own percentage calculation.
A Challenge
With the CSS calc() function in use, it would become difficult to adjust dynamically, as that function does not work for font-size purposes at this time. So you could not do a pure CSS adjustment if your width is changing on calc(). Of course, a minor adjustment of width for margins may not be enough to warrant any change in font-size, so it may not matter.

Solution with SVG:
.resizeme {
resize: both;
margin: 0;
padding: 0;
height: 75px;
width: 500px;
background-color: lightblue;
overflow: hidden;
}
<div class="resizeme">
<svg
width="100%"
height="100%"
viewBox="0 0 500 75"
preserveAspectRatio="xMinYMid meet"
style="background-color:green"
xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink"
>
<text
x="0"
y="75"
font-size="75"
fill="black"
>█Resize This█</text>
</svg>
</div>
Solution with SVG and text-wrapping using foreignObject:
.resizeme {
resize: both;
margin: 0;
padding: 0;
height: 200px;
width: 500px;
background-color: lightblue;
overflow: hidden;
}
<div class="resizeme">
<svg
width="100%"
height="100%"
viewBox="0 0 500 200"
preserveAspectRatio="xMinYMin meet"
>
<foreignObject width="100%" height="100%" xmlns="http://www.w3.org/1999/xhtml">
<div xmlns="http://www.w3.org/1999/xhtml" style="background-color:lightgreen;">
<h1>heading</h1>
<p>Resize the blue box.</p>
</div>
</foreignObject>
</svg>
</div>

In one of my projects I use a "mixture" between vw and vh to adjust the font size to my needs, for example:
font-size: calc(3vw + 3vh);
I know this doesn't answer the OP's question, but maybe it can be a solution to anyone else.

Pure-CSS solution with calc(), CSS units and math
This is precisely not what OP asks, but may make someone's day. This answer is not spoon-feedingly easy and needs some researching on the developer end.
I came finally to get a pure-CSS solution for this using calc() with different units. You will need some basic mathematical understanding of formulas to work out your expression for calc().
When I worked this out, I had to get a full-page-width responsive header with some padding few parents up in DOM. I'll use my values here, replace them with your own.
To mathematics
You will need:
Nicely adjusted ratio in some viewport. I used 320 pixels, thus I got 24 pixels high and 224 pixels wide, so the ratio is 9.333... or 28 / 3
The container width, I had padding: 3em and full width so this got to 100wv - 2 * 3em
X is the width of container, so replace it with your own expression or adjust the value to get full-page text. R is the ratio you will have. You can get it by adjusting the values in some viewport, inspecting element width and height and replacing them with your own values. Also, it is width / heigth ;)
x = 100vw - 2 * 3em = 100vw - 6em
r = 224px/24px = 9.333... = 28 / 3
y = x / r
= (100vw - 6em) / (28 / 3)
= (100vw - 6em) * 3 / 28
= (300vw - 18em) / 28
= (75vw - 4.5rem) / 7
And bang! It worked! I wrote
font-size: calc((75vw - 4.5rem) / 7)
to my header and it adjusted nicely in every viewport.
But how does it work?
We need some constants up here. 100vw means the full width of viewport, and my goal was to establish full-width header with some padding.
The ratio. Getting a width and height in one viewport got me a ratio to play with, and with ratio I know what the height should be in other viewport width. Calculating them with hand would take plenty of time and at least take lots of bandwidth, so it's not a good answer.
Conclusion
I wonder why no-one has figured this out and some people are even telling that this would be impossible to tinker with CSS. I don't like to use JavaScript in adjusting elements, so I don't accept JavaScript (and forget about jQuery) answers without digging more. All in all, it's good that this got figured out and this is one step to pure-CSS implementations in website design.
I apologize of any unusual convention in my text, I'm not native speaker in English and am also quite new to writing Stack Overflow answers.
It should also be noted that we have evil scrollbars in some browsers. For example, when using Firefox I noticed that 100vw means the full width of viewport, extending under scrollbar (where content cannot expand!), so the fullwidth text has to be margined carefully and preferably get tested with many browsers and devices.

There is a big philosophy for this issue.
The easiest thing to do would be to give a certain font-size to body (I recommend 10), and then all the other element would have their font in em or rem.
I'll give you an example to understand those units.
Em is always relative to its parent:
body{font-size: 10px;}
.menu{font-size: 2em;} /* That means 2*10 pixels = 20 pixels */
.menu li{font-size: 1.5em;} /* That means 1.5*20 pixels = 30 pixels */
Rem is always relative to body:
body{font-size: 10px;}
.menu{font-size: 2rem;} /* That means 2*10 pixels = 20 pixels */
.menu li{font-size: 1.5rem;} /* that means 1.5*10 pixels = 15 pixels */
And then you could create a script that would modify font-size relative to your container width.
But this isn't what I would recommend. Because in a 900 pixels width container for example you would have a p element with a 12 pixels font-size let's say. And on your idea that would become an 300 pixels wide container at 4 pixels font-size. There has to be a lower limit.
Other solutions would be with media queries, so that you could set font for different widths.
But the solutions that I would recommend is to use a JavaScript library that helps you with that. And fittext.js that I found so far.

Here is the function:
document.body.setScaledFont = function(f) {
var s = this.offsetWidth, fs = s * f;
this.style.fontSize = fs + '%';
return this
};
Then convert all your documents child element font sizes to em's or %.
Then add something like this to your code to set the base font size.
document.body.setScaledFont(0.35);
window.onresize = function() {
document.body.setScaledFont(0.35);
}
http://jsfiddle.net/0tpvccjt/

There is a way to do this without JavaScript!
You can use an inline SVG image. You can use CSS on an SVG if it is inline. You have to remember that using this method means your SVG image will respond to its container size.
Try using the following solution...
HTML
<div>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 360.96 358.98" >
<text>SAVE $500</text>
</svg>
</div>
CSS
div {
width: 50%; /* Set your container width */
height: 50%; /* Set your container height */
}
svg {
width: 100%;
height: auto;
}
text {
transform: translate(40px, 202px);
font-size: 62px;
fill: #000;
}
Example:
https://jsfiddle.net/k8L4xLLa/32/
Want something more flashy?
SVG images also allow you to do cool stuff with shapes and junk. Check out this great use case for scalable text...
https://jsfiddle.net/k8L4xLLa/14/

CSS Container Queries
A late-2022 addition to the CSS feature set makes scaling font size with containers straightforward.
Container queries come with a new set of CSS units cqw/cqh (container query width/height). To use them you need to set the container-type property on the parent element whose size you want to use. Minimal example:
<div>
<p>Lorem ipsum dolor sit amet</p>
</div>
<style>
div {
container-type: inline-size;
}
p {
font-size: 5cqw;
}
</style>
The font size will increase smoothly as the container grows. At 1000px container width, the p font size will be 1000px / 100 * 5 = 50px.
container-type can be size or inline-size. size tracks both height and width of the container which allows you to use both cqw and cqh.
Most of the time on the web, heights are calculated based on content and you only specify the width. To save the browser some work, you'll generally want to set container-type: inline-size; so the browser only tracks the inline dimension which is usually width (unless you set writing-mode to vertical).
Browser support for container queries has grown rapidly in the 2nd half of 2022 and currently stands at 75% (2023-01-01).

This may not be super practical, but if you want a font to be a direct function of the parent, without having any JavaScript that listens/loops (interval) to read the size of the div/page, there is a way to do it. Iframes.
Anything within the iframe will consider the size of the iframe as the size of the viewport. So the trick is to just make an iframe whose width is the maximum width you want your text to be, and whose height is equal to the maximum height * the particular text's aspect ratio.
Setting aside the limitation that viewport units can't also come along side parent units for text (as in, having the % size behave like everyone else), viewport units do provide a very powerful tool: being able to get the minimum/maximum dimension. You can't do that anywhere else - you can't say...make the height of this div be the width of the parent * something.
That being said, the trick is to use vmin, and to set the iframe size so that [fraction] * total height is a good font size when the height is the limiting dimension, and [fraction] * total width when the width is the limiting dimension. This is why the height has to be a product of the width and the aspect ratio.
For my particular example, you have
.main iframe{
position: absolute;
top: 50%;
left: 50%;
width: 100%;
height: calc(3.5 * 100%);
background: rgba(0, 0, 0, 0);
border-style: none;
transform: translate3d(-50%, -50%, 0);
}
The small annoyance with this method is that you have to manually set the CSS of the iframe. If you attach the whole CSS file, that would take up a lot of bandwidth for many text areas. So, what I do is attach the rule that I want directly from my CSS.
var rule = document.styleSheets[1].rules[4];
var iDoc = document.querySelector('iframe').contentDocument;
iDoc.styleSheets[0].insertRule(rule.cssText);
You can write small function that gets the CSS rule / all CSS rules that would affect the text area.
I cannot think of another way to do it without having some cycling/listening JavaScript. The real solution would be for browsers to provide a way to scale text as a function of the parent container and to also provide the same vmin/vmax type functionality.
JSFiddle:
https://jsfiddle.net/0jr7rrgm/3/
(click once to lock the red square to the mouse, and click again to release)
Most of the JavaScript in the fiddle is just my custom click-drag function.

Using vw, em & co. works for sure, but IMO it always needs a human's touch for fine-tuning.
Here's a script I just wrote based on #tnt-rox' answer that tries to automatize that human's touch:
$('#controller').click(function(){
$('h2').each(function(){
var
$el = $(this),
max = $el.get(0),
el = null
;
max =
max
? max.offsetWidth
: 320
;
$el.css({
'font-size': '1em',
'display': 'inline',
});
el = $el.get(0);
el.get_float = function(){
var
fs = 0
;
if (this.style && this.style.fontSize) {
fs = parseFloat(this.style.fontSize.replace(/([\d\.]+)em/g, '$1'));
}
return fs;
};
el.bigger = function(){
this.style.fontSize = (this.get_float() + 0.1) + 'em';
};
while (el.offsetWidth < max) {
el.bigger();
}
// Finishing touch.
$el.css({
'font-size': ((el.get_float() -0.1) +'em'),
'line-height': 'normal',
'display': '',
});
}); // end of (each)
}); // end of (font scaling test)
div {
width: 50%;
background-color: tomato;
font-family: 'Arial';
}
h2 {
white-space: nowrap;
}
h2:nth-child(2) {
font-style: italic;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<input type="button" id="controller" value="Apply" />
<div>
<h2>Lorem ipsum dolor</h2>
<h2>Test String</h2>
<h2>Sweet Concatenation</h2>
<h2>Font Scaling</h2>
</div>
It basically reduces the font-size to 1em and then starts incrementing by 0.1 until it reaches maximum width.
JSFiddle

Use CSS Variables
No one has mentioned CSS variables yet, and this approach worked best for me, so:
Let's say you've got a column on your page that is 100% of the width of a mobile user's screen, but has a max-width of 800px, so on desktop there's some space on either side of the column. Put this at the top of your page:
<script> document.documentElement.style.setProperty('--column-width', Math.min(window.innerWidth, 800)+'px'); </script>
And now you can use that variable (instead of the built-in vw unit) to set the size of your font. E.g.
p {
font-size: calc( var(--column-width) / 100 );
}
It's not a pure CSS approach, but it's pretty close.

100% is relative to the base font size, which, if you haven't set it, would be the browser's user-agent default.
To get the effect you're after, I would use a piece of JavaScript code to adjust the base font size relative to the window dimensions.

Artistically, if you need to fit two or more lines of text within the same width regardless of their character count then you have nice options.
It's best to find a dynamical solution so whatever text is entered we end up with a nice display.
Let's see how we may approach.
var els = document.querySelectorAll(".divtext"),
refWidth = els[0].clientWidth,
refFontSize = parseFloat(window.getComputedStyle(els[0],null)
.getPropertyValue("font-size"));
els.forEach((el,i) => el.style.fontSize = refFontSize * refWidth / els[i].clientWidth + "px")
#container {
display: inline-block;
background-color: black;
padding: 0.6vw 1.2vw;
}
.divtext {
display: table;
color: white;
font-family: impact;
font-size: 4.5vw;
}
<div id="container">
<div class="divtext">THIS IS JUST AN</div>
<div class="divtext">EXAMPLE</div>
<div class="divtext">TO SHOW YOU WHAT</div>
<div class="divtext">YOU WANT</div>
</div>
All we do is to get the width (els[0].clientWidth) and the font size (parseFloat(window.getComputedStyle(els[0],null).getPropertyValue("font-size"))) of the first line as a reference and then just calculate the subsequent lines font size accordingly.

This web component changes the font size so the inner text width matches the container width. Check the demo.
You can use it like this:
<full-width-text>Lorem Ipsum</full-width-text>

You may be you looking for something like this:
http://jsfiddle.net/sijav/dGsC9/4/
http://fiddle.jshell.net/sijav/dGsC9/4/show/
I have used flowtype, and it's working great (however it's JavaScript and not a pure CSS solution):
$('body').flowtype({
minFont: 10,
maxFont: 40,
minimum: 500,
maximum: 1200,
fontRatio: 70
});

I've prepared a simple scale function using CSS transform instead of font-size. You can use it inside of any container, you don't have to set media queries, etc. :)
Blog post:
Full width CSS & JS scalable header
The code:
function scaleHeader() {
var scalable = document.querySelectorAll('.scale--js');
var margin = 10;
for (var i = 0; i < scalable.length; i++) {
var scalableContainer = scalable[i].parentNode;
scalable[i].style.transform = 'scale(1)';
var scalableContainerWidth = scalableContainer.offsetWidth - margin;
var scalableWidth = scalable[i].offsetWidth;
scalable[i].style.transform = 'scale(' + scalableContainerWidth / scalableWidth + ')';
scalableContainer.style.height = scalable[i].getBoundingClientRect().height + 'px';
}
}
Working demo:
https://codepen.io/maciejkorsan/pen/BWLryj

Inside your CSS, try adding this at the bottom changing the 320 pixels width for wherever your design starts breaking:
#media only screen and (max-width: 320px) {
body { font-size: 1em; }
}
Then give the font-size in "px" or "em" as you wish.

Try http://simplefocus.com/flowtype/. This is what I use for my sites, and it has worked perfectly.

My own solution, jQuery-based, works by gradually increasing the font size until the container gets a big increase in height (meaning it got a line break).
It's pretty simple, but works fairly well, and it is very easy to use. You don't have to know anything about the font being used, everything is taken care of by the browser.
You can play with it on http://jsfiddle.net/tubededentifrice/u5y15d0L/2/
The magic happens here:
var setMaxTextSize=function(jElement) {
// Get and set the font size into data for reuse upon resize
var fontSize=parseInt(jElement.data(quickFitFontSizeData)) || parseInt(jElement.css("font-size"));
jElement.data(quickFitFontSizeData, fontSize);
// Gradually increase font size until the element gets a big increase in height (i.e. line break)
var i = 0;
var previousHeight;
do
{
previousHeight=jElement.height();
jElement.css("font-size", "" + (++fontSize) + "px");
}
while(i++ < 300 && jElement.height()-previousHeight < fontSize/2)
// Finally, go back before the increase in height and set the element as resized by adding quickFitSetClass
fontSize -= 1;
jElement.addClass(quickFitSetClass).css("font-size", "" + fontSize + "px");
return fontSize;
};

My problem was similar, but related to scaling text within a heading. I tried Fit Font, but I needed to toggle the compressor to get any results, since it was solving a slightly different problem, as was Text Flow.
So I wrote my own little plugin that reduced the font size to fit the container, assuming you have overflow: hidden and white-space: nowrap so that even if reducing the font to the minimum doesn't allow showing the full heading, it just cuts off what it can show.
(function($) {
// Reduces the size of text in the element to fit the parent.
$.fn.reduceTextSize = function(options) {
options = $.extend({
minFontSize: 10
}, options);
function checkWidth(em) {
var $em = $(em);
var oldPosition = $em.css('position');
$em.css('position', 'absolute');
var width = $em.width();
$em.css('position', oldPosition);
return width;
}
return this.each(function(){
var $this = $(this);
var $parent = $this.parent();
var prevFontSize;
while (checkWidth($this) > $parent.width()) {
var currentFontSize = parseInt($this.css('font-size').replace('px', ''));
// Stop looping if min font size reached, or font size did not change last iteration.
if (isNaN(currentFontSize) || currentFontSize <= options.minFontSize ||
prevFontSize && prevFontSize == currentFontSize) {
break;
}
prevFontSize = currentFontSize;
$this.css('font-size', (currentFontSize - 1) + 'px');
}
});
};
})(jQuery);

Try to use the fitText plugin, because Viewport sizes isn't the solution of this problem.
Just add the library:
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
And change font-size for correct by settings the coefficient of text:
$("#text_div").fitText(0.8);
You can set maximum and minimum values of text:
$("#text_div").fitText(0.8, { minFontSize: '12px', maxFontSize: '36px' });

Here is a pure CSS solution with the understanding that you admit breakpoints are necessary but also want text scaling:
I'm aware I can add breakpoints, but I really want the text to scale
as well as having extra breakpoints, otherwise....
Here is an approach using:
Custom properties
Media queries for breakpoints
clamp() (browser support in Feb 2022 is pretty good at 93%)
calc()
If one common scaling factor can be used to control ALL the text scaling within a container per screen max-width, all you need to do is scale a custom property per max-width, and apply this factor to 1 calculation.
A basic setup starts like this:
:root {
--scaling-factor: 1
}
.parent {
font-size: 30px
}
.largest {
font-size: clamp(60%, calc(var(--scaling-factor) * 100%), 100%);
}
.middle {
font-size: clamp(60%, calc(var(--scaling-factor) * 85%), 100%);
}
.smallest {
font-size: clamp(60%, calc(var(--scaling-factor) * 70%), 100%);
}
Then nest your media queries something like this (or whatever you need for your breakpoints):
#media (max-width: 1200px) {
:root {
--scaling-factor: 0.9
}
#media (max-width: 800px) {
:root {
--scaling-factor: 0.8
}
#media (max-width: 600px) {
:root {
--scaling-factor: 0.5 /* nope, because the font-size is floored at 60% thanks to clamp() */
}
}
}
}
This minimizes your media query markup.
Advantages
One custom property controls ALL scaling ... no need to add multiple declarations per media breakpoint
The use of clamp() sets a lower-limit on what the font-size should be, so you ensure your text is never too small (here the floor is 60% of the parent's font-size)
Please see this JSFiddle for a demo. Resize the window until at the smallest widths, the paragraphs are all the same font-size.

Always have your element with this attribute:
JavaScript: element.style.fontSize = "100%";
or
CSS: style = "font-size: 100%;"
When you go fullscreen, you should already have a scale variable calculated (scale > 1 or scale = 1). Then, on fullscreen:
document.body.style.fontSize = (scale * 100) + "%";
It works nicely with little code.

Take look at my code. It makes the font size smaller to fit whatever there.
But I think this doesn't lead to a good user experience
var containerWidth = $("#ui-id-2").width();
var items = $(".quickSearchAutocomplete .ui-menu-item");
var fontSize = 16;
items.each(function(){
// Displaying a value depends sometimes on your case. You may make it block or inline-table instead of inline-block or whatever value that make the div take overflow width.
$(this).css({"whiteSpace": "nowrap", "display": "inline-block"});
while ($(this).width() > containerWidth){
console.log("$(this).width()" + $(this).width() + "containerWidth" + containerWidth)
$(this).css("font-size", fontSize -= 0.5);
}
});

For dynamic text, this plugin is quite useful:
http://freqdec.github.io/slabText/
Simply add CSS:
.slabtexted .slabtext
{
display: -moz-inline-box;
display: inline-block;
white-space: nowrap;
}
.slabtextinactive .slabtext
{
display: inline;
white-space: normal;
font-size: 1em !important;
letter-spacing: inherit !important;
word-spacing: inherit !important;
*letter-spacing: normal !important;
*word-spacing: normal !important;
}
.slabtextdone .slabtext
{
display: block;
}
And the script:
$('#mydiv').slabText();

This worked for me:
I try to approximate font-size based on a width/height got from setting `font-size: 10px`. Basically, the idea is "if I have 20 pixels width and 11 pixels height with `font-size: 10px`, so what would it be the maximum font-size to math a container of 50 pixels width and 30 pixels height?"
The answer is a double proportion system:
{ 20:10=50:X, 11:10=30:Y } = { X= (10*50)/20, Y= (10*30)/11 }
Now X is a font-size that will match width, and Y is a font-size that will match height; take the smallest value
function getMaxFontSizeApprox(el){
var fontSize = 10;
var p = el.parentNode;
var parent_h = p.offsetHeight ? p.offsetHeight : p.style.pixelHeight;
if(!parent_h)
parent_h = 0;
var parent_w = p.offsetHeight ? p.offsetWidth : p.style.pixelWidth;
if(!parent_w)
parent_w = 0;
el.style.fontSize = fontSize + "px";
var el_h = el.offsetHeight ? el.offsetHeight : el.style.pixelHeight;
if(!el_h)
el_h = 0;
var el_w = el.offsetHeight ? el.offsetWidth : el.style.pixelWidth;
if(!el_w)
el_w = 0;
// 0.5 is the error on the measure that JavaScript does
// if the real measure had been 12.49 px => JavaScript would have said 12px
// so we think about the worst case when could have, we add 0.5 to
// compensate the round error
var fs1 = (fontSize*(parent_w + 0.5))/(el_w + 0.5);
var fs2 = (fontSize*(parent_h) + 0.5)/(el_h + 0.5);
fontSize = Math.floor(Math.min(fs1,fs2));
el.style.fontSize = fontSize + "px";
return fontSize;
}
NB: the argument of the function must be a span element or an element which is smaller than its parent, otherwise if children and parent have both the same width/height function will fail.

let textElement = document.getElementById('text1');
let parentElement = textElement.parentElement;
const parentClientHeight = parentElement.clientHeight;
const parentClientWidth = parentElement.clientWidth;
textElement.style.padding = "unset";
textElement.style.margin = "auto";
let fontSize = parentClientHeight;
let minFS = 3,
maxFS = fontSize;
while (fontSize != minFS) {
textElement.style.fontSize = `${fontSize}px`;
if (
parentElement.scrollHeight <= parentClientHeight &&
parentElement.scrollWidth <= parentClientWidth
) {
minFS = fontSize;
} else {
maxFS = fontSize;
}
fontSize = Math.floor((minFS + maxFS) / 2);
}
textElement.style.fontSize = `${minFS}px`;
<div style="height:200px; width:300px;">
<div id='text1'>
test
</div>
</div>

As a JavaScript fallback (or your sole solution), you can use my jQuery Scalem plugin, which lets you scale relative to the parent element (container) by passing the reference option.

In case it's helpful to anyone, most of the solutions in this thread were wrapping text into multiple lines, form e.
But then I found this, and it worked:
https://github.com/chunksnbits/jquery-quickfit
Example usage:
$('.someText').quickfit({max:50,tolerance:.4})

I'm having a hard time getting my head around font scaling.
I currently have a website with a body font-size of 100%. 100% of what though? This seems to compute out at 16 pixels.
I was under the impression that 100% would somehow refer to the size of the browser window, but apparently not because it's always 16 pixels whether the window is resized down to a mobile width or full-blown widescreen desktop.
How can I make the text on my site scale in relation to its container? I tried using em, but this doesn't scale either.
My reasoning is that things like my menu become squished when you resize, so I need to reduce the px font-size of .menuItem among other elements in relation to the width of the container. (For example, in the menu on a large desktop, 22px works perfectly. Move down to tablet width and 16px is more appropriate.)
I'm aware I can add breakpoints, but I really want the text to scale as well as having extra breakpoints, otherwise, I'll end up with hundreds of breakpoints for every 100pixels decrease in width to control the text.

If the container is not the body, CSS Tricks covers all of your options in Fitting Text to a Container.
If the container is the body, what you are looking for is Viewport-percentage lengths:
The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly. However, when the value of overflow on the root element is auto, any scroll bars are assumed not to exist.
The values are:
vw (% of the viewport width)
vh (% of the viewport height)
vi (1% of the viewport size in the direction of the root element's inline axis)
vb (1% of the viewport size in the direction of the root element's block axis)
vmin (the smaller of vw or vh)
vmax (the larger or vw or vh)
1 v* is equal to 1% of the initial containing block.
Using it looks like this:
p {
font-size: 4vw;
}
As you can see, when the viewport width increases, so do the font-size, without needing to use media queries.
These values are a sizing unit, just like px or em, so they can be used to size other elements as well, such as width, margin, or padding.
Browser support is pretty good, but you'll likely need a fallback, such as:
p {
font-size: 16px;
font-size: 4vw;
}
Check out the support statistics: http://caniuse.com/#feat=viewport-units.
Also, check out CSS-Tricks for a broader look: Viewport Sized Typography
Here's a nice article about setting minimum/maximum sizes and exercising a bit more control over the sizes: Precise control over responsive typography
And here's an article about setting your size using calc() so that the text fills the viewport: http://codepen.io/CrocoDillon/pen/fBJxu
Also, please view this article, which uses a technique dubbed 'molten leading' to adjust the line-height as well. Molten Leading in CSS

But what if the container is not the viewport (body)?
This question is asked in a comment by Alex under 2507rkt3's answer.
That fact does not mean vw cannot be used to some extent to size for that container. Now to see any variation at all one has to be assuming that the container in some way is flexible in size. Whether through a direct percentage width or through being 100% minus margins. The point becomes "moot" if the container is always set to, let's say, 200px wide--then just set a font-size that works for that width.
Example 1
With a flexible width container, however, it must be realized that in some way the container is still being sized off the viewport. As such, it is a matter of adjusting a vw setting based off that percentage size difference to the viewport, which means taking into account the sizing of parent wrappers. Take this example:
div {
width: 50%;
border: 1px solid black;
margin: 20px;
font-size: 16px;
/* 100 = viewport width, as 1vw = 1/100th of that
So if the container is 50% of viewport (as here)
then factor that into how you want it to size.
Let's say you like 5vw if it were the whole width,
then for this container, size it at 2.5vw (5 * .5 [i.e. 50%])
*/
font-size: 2.5vw;
}
Assuming here the div is a child of the body, it is 50% of that 100% width, which is the viewport size in this basic case. Basically, you want to set a vw that is going to look good to you. As you can see in my comment in the above CSS content, you can "think" through that mathematically with respect to the full viewport size, but you don't need to do that. The text is going to "flex" with the container because the container is flexing with the viewport resizing. Here's an example of two differently sized containers.
Example 2
You can help ensure viewport sizing by forcing the calculation based off that. Consider this example:
html {width: 100%;} /* Force 'html' to be viewport width */
body {width: 150%; } /* Overflow the body */
div {
width: 50%;
border: 1px solid black;
margin: 20px;
font-size: 16px;
/* 100 = viewport width, as 1vw = 1/100th of that
Here, the body is 150% of viewport, but the container is 50%
of viewport, so both parents factor into how you want it to size.
Let's say you like 5vw if it were the whole width,
then for this container, size it at 3.75vw
(5 * 1.5 [i.e. 150%]) * .5 [i.e. 50%]
*/
font-size: 3.75vw;
}
The sizing is still based off viewport, but is in essence set up based off the container size itself.
Should the Size of the Container Change Dynamically...
If the sizing of the container element ended up changing dynamically its percentage relationship either via #media breakpoints or via JavaScript, then whatever the base "target" was would need recalculation to maintain the same "relationship" for text sizing.
Take example #1 above. If the div was switched to 25% width by either #media or JavaScript, then at the same time, the font-size would need to adjust in either the media query or by JavaScript to the new calculation of 5vw * .25 = 1.25. This would put the text size at the same size it would have been had the "width" of the original 50% container been reduced by half from viewport sizing, but has now been reduced due to a change in its own percentage calculation.
A Challenge
With the CSS calc() function in use, it would become difficult to adjust dynamically, as that function does not work for font-size purposes at this time. So you could not do a pure CSS adjustment if your width is changing on calc(). Of course, a minor adjustment of width for margins may not be enough to warrant any change in font-size, so it may not matter.

Solution with SVG:
.resizeme {
resize: both;
margin: 0;
padding: 0;
height: 75px;
width: 500px;
background-color: lightblue;
overflow: hidden;
}
<div class="resizeme">
<svg
width="100%"
height="100%"
viewBox="0 0 500 75"
preserveAspectRatio="xMinYMid meet"
style="background-color:green"
xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink"
>
<text
x="0"
y="75"
font-size="75"
fill="black"
>█Resize This█</text>
</svg>
</div>
Solution with SVG and text-wrapping using foreignObject:
.resizeme {
resize: both;
margin: 0;
padding: 0;
height: 200px;
width: 500px;
background-color: lightblue;
overflow: hidden;
}
<div class="resizeme">
<svg
width="100%"
height="100%"
viewBox="0 0 500 200"
preserveAspectRatio="xMinYMin meet"
>
<foreignObject width="100%" height="100%" xmlns="http://www.w3.org/1999/xhtml">
<div xmlns="http://www.w3.org/1999/xhtml" style="background-color:lightgreen;">
<h1>heading</h1>
<p>Resize the blue box.</p>
</div>
</foreignObject>
</svg>
</div>

In one of my projects I use a "mixture" between vw and vh to adjust the font size to my needs, for example:
font-size: calc(3vw + 3vh);
I know this doesn't answer the OP's question, but maybe it can be a solution to anyone else.

Pure-CSS solution with calc(), CSS units and math
This is precisely not what OP asks, but may make someone's day. This answer is not spoon-feedingly easy and needs some researching on the developer end.
I came finally to get a pure-CSS solution for this using calc() with different units. You will need some basic mathematical understanding of formulas to work out your expression for calc().
When I worked this out, I had to get a full-page-width responsive header with some padding few parents up in DOM. I'll use my values here, replace them with your own.
To mathematics
You will need:
Nicely adjusted ratio in some viewport. I used 320 pixels, thus I got 24 pixels high and 224 pixels wide, so the ratio is 9.333... or 28 / 3
The container width, I had padding: 3em and full width so this got to 100wv - 2 * 3em
X is the width of container, so replace it with your own expression or adjust the value to get full-page text. R is the ratio you will have. You can get it by adjusting the values in some viewport, inspecting element width and height and replacing them with your own values. Also, it is width / heigth ;)
x = 100vw - 2 * 3em = 100vw - 6em
r = 224px/24px = 9.333... = 28 / 3
y = x / r
= (100vw - 6em) / (28 / 3)
= (100vw - 6em) * 3 / 28
= (300vw - 18em) / 28
= (75vw - 4.5rem) / 7
And bang! It worked! I wrote
font-size: calc((75vw - 4.5rem) / 7)
to my header and it adjusted nicely in every viewport.
But how does it work?
We need some constants up here. 100vw means the full width of viewport, and my goal was to establish full-width header with some padding.
The ratio. Getting a width and height in one viewport got me a ratio to play with, and with ratio I know what the height should be in other viewport width. Calculating them with hand would take plenty of time and at least take lots of bandwidth, so it's not a good answer.
Conclusion
I wonder why no-one has figured this out and some people are even telling that this would be impossible to tinker with CSS. I don't like to use JavaScript in adjusting elements, so I don't accept JavaScript (and forget about jQuery) answers without digging more. All in all, it's good that this got figured out and this is one step to pure-CSS implementations in website design.
I apologize of any unusual convention in my text, I'm not native speaker in English and am also quite new to writing Stack Overflow answers.
It should also be noted that we have evil scrollbars in some browsers. For example, when using Firefox I noticed that 100vw means the full width of viewport, extending under scrollbar (where content cannot expand!), so the fullwidth text has to be margined carefully and preferably get tested with many browsers and devices.

There is a big philosophy for this issue.
The easiest thing to do would be to give a certain font-size to body (I recommend 10), and then all the other element would have their font in em or rem.
I'll give you an example to understand those units.
Em is always relative to its parent:
body{font-size: 10px;}
.menu{font-size: 2em;} /* That means 2*10 pixels = 20 pixels */
.menu li{font-size: 1.5em;} /* That means 1.5*20 pixels = 30 pixels */
Rem is always relative to body:
body{font-size: 10px;}
.menu{font-size: 2rem;} /* That means 2*10 pixels = 20 pixels */
.menu li{font-size: 1.5rem;} /* that means 1.5*10 pixels = 15 pixels */
And then you could create a script that would modify font-size relative to your container width.
But this isn't what I would recommend. Because in a 900 pixels width container for example you would have a p element with a 12 pixels font-size let's say. And on your idea that would become an 300 pixels wide container at 4 pixels font-size. There has to be a lower limit.
Other solutions would be with media queries, so that you could set font for different widths.
But the solutions that I would recommend is to use a JavaScript library that helps you with that. And fittext.js that I found so far.

Here is the function:
document.body.setScaledFont = function(f) {
var s = this.offsetWidth, fs = s * f;
this.style.fontSize = fs + '%';
return this
};
Then convert all your documents child element font sizes to em's or %.
Then add something like this to your code to set the base font size.
document.body.setScaledFont(0.35);
window.onresize = function() {
document.body.setScaledFont(0.35);
}
http://jsfiddle.net/0tpvccjt/

There is a way to do this without JavaScript!
You can use an inline SVG image. You can use CSS on an SVG if it is inline. You have to remember that using this method means your SVG image will respond to its container size.
Try using the following solution...
HTML
<div>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 360.96 358.98" >
<text>SAVE $500</text>
</svg>
</div>
CSS
div {
width: 50%; /* Set your container width */
height: 50%; /* Set your container height */
}
svg {
width: 100%;
height: auto;
}
text {
transform: translate(40px, 202px);
font-size: 62px;
fill: #000;
}
Example:
https://jsfiddle.net/k8L4xLLa/32/
Want something more flashy?
SVG images also allow you to do cool stuff with shapes and junk. Check out this great use case for scalable text...
https://jsfiddle.net/k8L4xLLa/14/

CSS Container Queries
A late-2022 addition to the CSS feature set makes scaling font size with containers straightforward.
Container queries come with a new set of CSS units cqw/cqh (container query width/height). To use them you need to set the container-type property on the parent element whose size you want to use. Minimal example:
<div>
<p>Lorem ipsum dolor sit amet</p>
</div>
<style>
div {
container-type: inline-size;
}
p {
font-size: 5cqw;
}
</style>
The font size will increase smoothly as the container grows. At 1000px container width, the p font size will be 1000px / 100 * 5 = 50px.
container-type can be size or inline-size. size tracks both height and width of the container which allows you to use both cqw and cqh.
Most of the time on the web, heights are calculated based on content and you only specify the width. To save the browser some work, you'll generally want to set container-type: inline-size; so the browser only tracks the inline dimension which is usually width (unless you set writing-mode to vertical).
Browser support for container queries has grown rapidly in the 2nd half of 2022 and currently stands at 75% (2023-01-01).

This may not be super practical, but if you want a font to be a direct function of the parent, without having any JavaScript that listens/loops (interval) to read the size of the div/page, there is a way to do it. Iframes.
Anything within the iframe will consider the size of the iframe as the size of the viewport. So the trick is to just make an iframe whose width is the maximum width you want your text to be, and whose height is equal to the maximum height * the particular text's aspect ratio.
Setting aside the limitation that viewport units can't also come along side parent units for text (as in, having the % size behave like everyone else), viewport units do provide a very powerful tool: being able to get the minimum/maximum dimension. You can't do that anywhere else - you can't say...make the height of this div be the width of the parent * something.
That being said, the trick is to use vmin, and to set the iframe size so that [fraction] * total height is a good font size when the height is the limiting dimension, and [fraction] * total width when the width is the limiting dimension. This is why the height has to be a product of the width and the aspect ratio.
For my particular example, you have
.main iframe{
position: absolute;
top: 50%;
left: 50%;
width: 100%;
height: calc(3.5 * 100%);
background: rgba(0, 0, 0, 0);
border-style: none;
transform: translate3d(-50%, -50%, 0);
}
The small annoyance with this method is that you have to manually set the CSS of the iframe. If you attach the whole CSS file, that would take up a lot of bandwidth for many text areas. So, what I do is attach the rule that I want directly from my CSS.
var rule = document.styleSheets[1].rules[4];
var iDoc = document.querySelector('iframe').contentDocument;
iDoc.styleSheets[0].insertRule(rule.cssText);
You can write small function that gets the CSS rule / all CSS rules that would affect the text area.
I cannot think of another way to do it without having some cycling/listening JavaScript. The real solution would be for browsers to provide a way to scale text as a function of the parent container and to also provide the same vmin/vmax type functionality.
JSFiddle:
https://jsfiddle.net/0jr7rrgm/3/
(click once to lock the red square to the mouse, and click again to release)
Most of the JavaScript in the fiddle is just my custom click-drag function.

Using vw, em & co. works for sure, but IMO it always needs a human's touch for fine-tuning.
Here's a script I just wrote based on #tnt-rox' answer that tries to automatize that human's touch:
$('#controller').click(function(){
$('h2').each(function(){
var
$el = $(this),
max = $el.get(0),
el = null
;
max =
max
? max.offsetWidth
: 320
;
$el.css({
'font-size': '1em',
'display': 'inline',
});
el = $el.get(0);
el.get_float = function(){
var
fs = 0
;
if (this.style && this.style.fontSize) {
fs = parseFloat(this.style.fontSize.replace(/([\d\.]+)em/g, '$1'));
}
return fs;
};
el.bigger = function(){
this.style.fontSize = (this.get_float() + 0.1) + 'em';
};
while (el.offsetWidth < max) {
el.bigger();
}
// Finishing touch.
$el.css({
'font-size': ((el.get_float() -0.1) +'em'),
'line-height': 'normal',
'display': '',
});
}); // end of (each)
}); // end of (font scaling test)
div {
width: 50%;
background-color: tomato;
font-family: 'Arial';
}
h2 {
white-space: nowrap;
}
h2:nth-child(2) {
font-style: italic;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<input type="button" id="controller" value="Apply" />
<div>
<h2>Lorem ipsum dolor</h2>
<h2>Test String</h2>
<h2>Sweet Concatenation</h2>
<h2>Font Scaling</h2>
</div>
It basically reduces the font-size to 1em and then starts incrementing by 0.1 until it reaches maximum width.
JSFiddle

Use CSS Variables
No one has mentioned CSS variables yet, and this approach worked best for me, so:
Let's say you've got a column on your page that is 100% of the width of a mobile user's screen, but has a max-width of 800px, so on desktop there's some space on either side of the column. Put this at the top of your page:
<script> document.documentElement.style.setProperty('--column-width', Math.min(window.innerWidth, 800)+'px'); </script>
And now you can use that variable (instead of the built-in vw unit) to set the size of your font. E.g.
p {
font-size: calc( var(--column-width) / 100 );
}
It's not a pure CSS approach, but it's pretty close.

100% is relative to the base font size, which, if you haven't set it, would be the browser's user-agent default.
To get the effect you're after, I would use a piece of JavaScript code to adjust the base font size relative to the window dimensions.

Artistically, if you need to fit two or more lines of text within the same width regardless of their character count then you have nice options.
It's best to find a dynamical solution so whatever text is entered we end up with a nice display.
Let's see how we may approach.
var els = document.querySelectorAll(".divtext"),
refWidth = els[0].clientWidth,
refFontSize = parseFloat(window.getComputedStyle(els[0],null)
.getPropertyValue("font-size"));
els.forEach((el,i) => el.style.fontSize = refFontSize * refWidth / els[i].clientWidth + "px")
#container {
display: inline-block;
background-color: black;
padding: 0.6vw 1.2vw;
}
.divtext {
display: table;
color: white;
font-family: impact;
font-size: 4.5vw;
}
<div id="container">
<div class="divtext">THIS IS JUST AN</div>
<div class="divtext">EXAMPLE</div>
<div class="divtext">TO SHOW YOU WHAT</div>
<div class="divtext">YOU WANT</div>
</div>
All we do is to get the width (els[0].clientWidth) and the font size (parseFloat(window.getComputedStyle(els[0],null).getPropertyValue("font-size"))) of the first line as a reference and then just calculate the subsequent lines font size accordingly.

This web component changes the font size so the inner text width matches the container width. Check the demo.
You can use it like this:
<full-width-text>Lorem Ipsum</full-width-text>

You may be you looking for something like this:
http://jsfiddle.net/sijav/dGsC9/4/
http://fiddle.jshell.net/sijav/dGsC9/4/show/
I have used flowtype, and it's working great (however it's JavaScript and not a pure CSS solution):
$('body').flowtype({
minFont: 10,
maxFont: 40,
minimum: 500,
maximum: 1200,
fontRatio: 70
});

I've prepared a simple scale function using CSS transform instead of font-size. You can use it inside of any container, you don't have to set media queries, etc. :)
Blog post:
Full width CSS & JS scalable header
The code:
function scaleHeader() {
var scalable = document.querySelectorAll('.scale--js');
var margin = 10;
for (var i = 0; i < scalable.length; i++) {
var scalableContainer = scalable[i].parentNode;
scalable[i].style.transform = 'scale(1)';
var scalableContainerWidth = scalableContainer.offsetWidth - margin;
var scalableWidth = scalable[i].offsetWidth;
scalable[i].style.transform = 'scale(' + scalableContainerWidth / scalableWidth + ')';
scalableContainer.style.height = scalable[i].getBoundingClientRect().height + 'px';
}
}
Working demo:
https://codepen.io/maciejkorsan/pen/BWLryj

Inside your CSS, try adding this at the bottom changing the 320 pixels width for wherever your design starts breaking:
#media only screen and (max-width: 320px) {
body { font-size: 1em; }
}
Then give the font-size in "px" or "em" as you wish.

Try http://simplefocus.com/flowtype/. This is what I use for my sites, and it has worked perfectly.

My own solution, jQuery-based, works by gradually increasing the font size until the container gets a big increase in height (meaning it got a line break).
It's pretty simple, but works fairly well, and it is very easy to use. You don't have to know anything about the font being used, everything is taken care of by the browser.
You can play with it on http://jsfiddle.net/tubededentifrice/u5y15d0L/2/
The magic happens here:
var setMaxTextSize=function(jElement) {
// Get and set the font size into data for reuse upon resize
var fontSize=parseInt(jElement.data(quickFitFontSizeData)) || parseInt(jElement.css("font-size"));
jElement.data(quickFitFontSizeData, fontSize);
// Gradually increase font size until the element gets a big increase in height (i.e. line break)
var i = 0;
var previousHeight;
do
{
previousHeight=jElement.height();
jElement.css("font-size", "" + (++fontSize) + "px");
}
while(i++ < 300 && jElement.height()-previousHeight < fontSize/2)
// Finally, go back before the increase in height and set the element as resized by adding quickFitSetClass
fontSize -= 1;
jElement.addClass(quickFitSetClass).css("font-size", "" + fontSize + "px");
return fontSize;
};

My problem was similar, but related to scaling text within a heading. I tried Fit Font, but I needed to toggle the compressor to get any results, since it was solving a slightly different problem, as was Text Flow.
So I wrote my own little plugin that reduced the font size to fit the container, assuming you have overflow: hidden and white-space: nowrap so that even if reducing the font to the minimum doesn't allow showing the full heading, it just cuts off what it can show.
(function($) {
// Reduces the size of text in the element to fit the parent.
$.fn.reduceTextSize = function(options) {
options = $.extend({
minFontSize: 10
}, options);
function checkWidth(em) {
var $em = $(em);
var oldPosition = $em.css('position');
$em.css('position', 'absolute');
var width = $em.width();
$em.css('position', oldPosition);
return width;
}
return this.each(function(){
var $this = $(this);
var $parent = $this.parent();
var prevFontSize;
while (checkWidth($this) > $parent.width()) {
var currentFontSize = parseInt($this.css('font-size').replace('px', ''));
// Stop looping if min font size reached, or font size did not change last iteration.
if (isNaN(currentFontSize) || currentFontSize <= options.minFontSize ||
prevFontSize && prevFontSize == currentFontSize) {
break;
}
prevFontSize = currentFontSize;
$this.css('font-size', (currentFontSize - 1) + 'px');
}
});
};
})(jQuery);

Try to use the fitText plugin, because Viewport sizes isn't the solution of this problem.
Just add the library:
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
And change font-size for correct by settings the coefficient of text:
$("#text_div").fitText(0.8);
You can set maximum and minimum values of text:
$("#text_div").fitText(0.8, { minFontSize: '12px', maxFontSize: '36px' });

Here is a pure CSS solution with the understanding that you admit breakpoints are necessary but also want text scaling:
I'm aware I can add breakpoints, but I really want the text to scale
as well as having extra breakpoints, otherwise....
Here is an approach using:
Custom properties
Media queries for breakpoints
clamp() (browser support in Feb 2022 is pretty good at 93%)
calc()
If one common scaling factor can be used to control ALL the text scaling within a container per screen max-width, all you need to do is scale a custom property per max-width, and apply this factor to 1 calculation.
A basic setup starts like this:
:root {
--scaling-factor: 1
}
.parent {
font-size: 30px
}
.largest {
font-size: clamp(60%, calc(var(--scaling-factor) * 100%), 100%);
}
.middle {
font-size: clamp(60%, calc(var(--scaling-factor) * 85%), 100%);
}
.smallest {
font-size: clamp(60%, calc(var(--scaling-factor) * 70%), 100%);
}
Then nest your media queries something like this (or whatever you need for your breakpoints):
#media (max-width: 1200px) {
:root {
--scaling-factor: 0.9
}
#media (max-width: 800px) {
:root {
--scaling-factor: 0.8
}
#media (max-width: 600px) {
:root {
--scaling-factor: 0.5 /* nope, because the font-size is floored at 60% thanks to clamp() */
}
}
}
}
This minimizes your media query markup.
Advantages
One custom property controls ALL scaling ... no need to add multiple declarations per media breakpoint
The use of clamp() sets a lower-limit on what the font-size should be, so you ensure your text is never too small (here the floor is 60% of the parent's font-size)
Please see this JSFiddle for a demo. Resize the window until at the smallest widths, the paragraphs are all the same font-size.

Always have your element with this attribute:
JavaScript: element.style.fontSize = "100%";
or
CSS: style = "font-size: 100%;"
When you go fullscreen, you should already have a scale variable calculated (scale > 1 or scale = 1). Then, on fullscreen:
document.body.style.fontSize = (scale * 100) + "%";
It works nicely with little code.

Take look at my code. It makes the font size smaller to fit whatever there.
But I think this doesn't lead to a good user experience
var containerWidth = $("#ui-id-2").width();
var items = $(".quickSearchAutocomplete .ui-menu-item");
var fontSize = 16;
items.each(function(){
// Displaying a value depends sometimes on your case. You may make it block or inline-table instead of inline-block or whatever value that make the div take overflow width.
$(this).css({"whiteSpace": "nowrap", "display": "inline-block"});
while ($(this).width() > containerWidth){
console.log("$(this).width()" + $(this).width() + "containerWidth" + containerWidth)
$(this).css("font-size", fontSize -= 0.5);
}
});

For dynamic text, this plugin is quite useful:
http://freqdec.github.io/slabText/
Simply add CSS:
.slabtexted .slabtext
{
display: -moz-inline-box;
display: inline-block;
white-space: nowrap;
}
.slabtextinactive .slabtext
{
display: inline;
white-space: normal;
font-size: 1em !important;
letter-spacing: inherit !important;
word-spacing: inherit !important;
*letter-spacing: normal !important;
*word-spacing: normal !important;
}
.slabtextdone .slabtext
{
display: block;
}
And the script:
$('#mydiv').slabText();

This worked for me:
I try to approximate font-size based on a width/height got from setting `font-size: 10px`. Basically, the idea is "if I have 20 pixels width and 11 pixels height with `font-size: 10px`, so what would it be the maximum font-size to math a container of 50 pixels width and 30 pixels height?"
The answer is a double proportion system:
{ 20:10=50:X, 11:10=30:Y } = { X= (10*50)/20, Y= (10*30)/11 }
Now X is a font-size that will match width, and Y is a font-size that will match height; take the smallest value
function getMaxFontSizeApprox(el){
var fontSize = 10;
var p = el.parentNode;
var parent_h = p.offsetHeight ? p.offsetHeight : p.style.pixelHeight;
if(!parent_h)
parent_h = 0;
var parent_w = p.offsetHeight ? p.offsetWidth : p.style.pixelWidth;
if(!parent_w)
parent_w = 0;
el.style.fontSize = fontSize + "px";
var el_h = el.offsetHeight ? el.offsetHeight : el.style.pixelHeight;
if(!el_h)
el_h = 0;
var el_w = el.offsetHeight ? el.offsetWidth : el.style.pixelWidth;
if(!el_w)
el_w = 0;
// 0.5 is the error on the measure that JavaScript does
// if the real measure had been 12.49 px => JavaScript would have said 12px
// so we think about the worst case when could have, we add 0.5 to
// compensate the round error
var fs1 = (fontSize*(parent_w + 0.5))/(el_w + 0.5);
var fs2 = (fontSize*(parent_h) + 0.5)/(el_h + 0.5);
fontSize = Math.floor(Math.min(fs1,fs2));
el.style.fontSize = fontSize + "px";
return fontSize;
}
NB: the argument of the function must be a span element or an element which is smaller than its parent, otherwise if children and parent have both the same width/height function will fail.

let textElement = document.getElementById('text1');
let parentElement = textElement.parentElement;
const parentClientHeight = parentElement.clientHeight;
const parentClientWidth = parentElement.clientWidth;
textElement.style.padding = "unset";
textElement.style.margin = "auto";
let fontSize = parentClientHeight;
let minFS = 3,
maxFS = fontSize;
while (fontSize != minFS) {
textElement.style.fontSize = `${fontSize}px`;
if (
parentElement.scrollHeight <= parentClientHeight &&
parentElement.scrollWidth <= parentClientWidth
) {
minFS = fontSize;
} else {
maxFS = fontSize;
}
fontSize = Math.floor((minFS + maxFS) / 2);
}
textElement.style.fontSize = `${minFS}px`;
<div style="height:200px; width:300px;">
<div id='text1'>
test
</div>
</div>

As a JavaScript fallback (or your sole solution), you can use my jQuery Scalem plugin, which lets you scale relative to the parent element (container) by passing the reference option.

In case it's helpful to anyone, most of the solutions in this thread were wrapping text into multiple lines, form e.
But then I found this, and it worked:
https://github.com/chunksnbits/jquery-quickfit
Example usage:
$('.someText').quickfit({max:50,tolerance:.4})