Develop Reference

Resolve URL Contents Into HTML Head As Text

I created a plugin for some web software that doesn't natively support plugins. I found that I could inject code into the head of the plugin webpage through their GUI. Currently, users have to paste a huge block of code into the head which contains the plugin HTML, CSS, and JS all minified into one block. I'd like to instead serve this over a CDN so users could simply subscribe and receive all updates without having to repaste. It also would keep them from having to paste a large chunk of code into the tool and instead it would be a nice link.
I tried:
<link rel="import" href="~my_CDN_link~">
but it seemed to only partially work. I didn't really understand the issue, but I saw that import is deprecated so this seems like it would only be a temporary patch.
At the end of the CDN link is a text file which contains:
HTML
<script>~my_scripts~</script>
<style>~my_styles~</style>
I was hoping there is some way to just resolve this link into the text which it contains and "paste" it into the head of the document. Maybe this isn't even the approach I should be taking? Should I instead provide two links? One for the stylesheet and one from the JavaScript? The JavaScript could then make a call to get the required markup and append it to the document. I'd love to keep it as one link if at all possible.

How to minimize load time of a landing page

I'm honestly confused about where to start with minimizing the load time of a website i'm building - https://projectrhea.herokuapp.com/ . Currently takes around 9 seconds to load the website which I want to try and bring down to sub 3 seconds.
I've done a diagnostic test, shown here https://www.webpagetest.org/result/171113_T2_851758db144ac117ab4e986a3798b1b5/1/details/#waterfall_view_step1 .
From what I can see there are three main reasons it would be taking awhile to load.
The first is the javascript.
I only need a small amount of it to run the site but I am very
confused about how to separate the code I need from the code I don't
need. I use it for the banner to show multiple phrases underneath the
banner that I would rather keep.
The second part is the shear amount of CSS files I'm drawing from.
I used a template to begin the site (it was a good way for me to learn
how to design the site). Now I think this has meant there is way too
many css and other files connected to this landing page.
The third part is the video file I have.
I would love to keep this
video as I just like how it fits in the site. I'll try and minimize
the file size after I have worked out the above issues.
This is my first real time trying to solve an issue like this and I would really appreciate the knowledge a more experienced coder could bring to this. Thanks!

Use following automated tools:
https://developers.google.com/speed/pagespeed/
https://gtmetrix.com/
Chrome's lighthouse
Optimize image sizes and quality. (Automated tools above provide You with the optimized images)
Place Your CSS file at the beginning of your bootstrapping file such as index.html
Compress CSS files (remove the formatting)
Place Your JS file at the bottom of the file.
Compress JS files (remove the formatting)

Trying to address the specific points raised by you.
The first is the javascript.
I only need a small amount of it to run the site but I am very
confused about how to separate the code I need from the code I don't
need. I use it for the banner to show multiple phrases underneath the
banner that I would rather keep.
Your JS files are not minified. Please make sure you are minifying your js files and order as suggested by Ante Jablan Adamović.
The second part is the shear amount of CSS files I'm drawing from.
I used a template to begin the site (it was a good way for me to learn
how to design the site). Now I think this has meant there is way too
many css and other files connected to this landing page.
You should combine and minify all the CSS files.
For minification and combining of JS and CSS you can use gulp.
https://github.com/gulpjs/gulp
The third part is the video file I have.
I would love to keep this video as I just like how it fits in the
site. I'll try and minimize the file size after I have worked out the
above issues.
I can see that you are serving some resources through S3. See if you can move your video as well to S3 and serve it through cloudfront.

Place your css imports in the top of the page and import your javascript after your html body ends.

Well for a start -
Place the css at the top, consider using a js loader to load the js once the page has loaded.
If you can obviously remove everything you don't need their are tools to do this but in reality is a tough task especially if its a template
And finally with the video get an image of the first frame, show the image not the video when the page loads.
How to make a loading image when loading HTML5 video?

The bigger your css, the longer the page takes to load. So try to reduce/minify css and try to use css in a single file. Same with JS also
Use Lazy load for images so webpage displays quickly without calling images.
Make sure server is using keep-alive as it can truly affect how your server fulfills requests.
Enable gzip compression
Minimize page redirects because it affect page speed
Enable browser caching so your browser can load the page without having to send another HTTP request to the server.

HTML : load thumbnail image and full image faster

I have a blog website that loads the images slowly i want to know how to make them load faster and:
I am using same image for thumbnail and story. thumbnail is small,does it still load full image?if so how to use thumbnail of an image?
Where should i store the images? what is the best location to store images for your websites and blogs? can save them in one drive and use the source?
how to optimise images?what is a placeholder?i have seen many websites such as facebook use a kind of place holder which displays before image and content?how to do so?
-how to i preload images ? or is there any better way ?

Here are some pointers.
Thumbnail images have to be separate from original (large) images. When the user uploads the images, you have to use some script to resize the images. If you are using a standard CMS like Drupal or Wordpress, there should be an option somewhere to do the resizing (without you having to write code).
Assuming your blog is public, the images as well should be public (usually). You can create a directory named files and you can store the images inside that directory. If you are using a standard CMS, these options should be there in some form.
To avoid having all files in one directory in the long run, use folder naming schemes like files/[YEAR]/[MONTH] or anything else you think would serve your purpose.
Make sure the uploads directory and your upload mechanism is well-protected using and .htaccess (or equivalent). Otherwise, someone might upload malicious scripts and execute them on your server.
A placeholder is anything which holds the place of something while the original thing is absent (or being loaded). So, a placeholder image will be a standard image with a general design - it's as good as saying loading. You can use JavaScript or CSS (background-image) to achieve such a placeholder.
Preloading should not be necessary as far as I see from your question. A better opinion / answer could be given if you share the link to your site.
Next time, please try to make detailed questions - one question per problem, if possible. Also, do not fear to Google for a solution. I learnt programming (PHP, JS, Drupal, CodeIgniter and more) just by Googling! Hope this helps!

Jigar has done a fairly good job of answering the question though I thought I'd add if you want to optimise images there are plenty of websites that do it for you for free.
My favourite is https://tinyjpg.com/ however there are plenty of others. A quick Google search will get you plenty of different sites all doing basically the same thing.
This post might also help Load a low-res background image first, then a high-res one

How to embed HTML5/CSS + js music player on single wordpress page?

I have an html 5, css, and javascript audio player that I'm trying to add to one page on my wordpress site.
I tried to paste the html into my page's text editor, while installing the JS and CSS folders for the player in my child theme's folder, which didn't work.
I tried placing the CSS for the player styling in my child themes CSS file, and that didn't work. I also tried linking to the style sheets and js scripts from the header.php file, which wasn't working for me.
My first thoughts are that because the html is showing up as it should, the css and JS files aren't being called. So I just need to figure out the path structure of how wordpress creates pages? Yet I know wordpress does something with a database to display the pages?
I saw some documentation about using I think it was either php or JS functions to call scripts or something and I don't understand enough of those languages to be able to make sense of it yet.
I'm still working with my developer, but he's 7 hours ahead of me and asleep. I just want to put the hardcoded version of the player and a few songs on my site for now so I can do some testing/see how it looks.
I hope that this question meets the SO req's. I tried to search on my own and I know the answer is out there, I just don't know enough to understand it yet. Trying to get a head start

The path to your Child theme's folder should be '/wp-content/themes/name-of-theme/'. You can use that in your header to call the files from wherever you might have placed them in your Child theme's folder.
To hard code the HTML, pasting it in the WordPress page editor should work fine unless there is another JS call or some weird iFraming going on.. if that's the case, you can simply create a Page Template (https://developer.wordpress.org/themes/template-files-section/page-template-files/page-templates/#creating-custom-page-templates-for-global-use) and paste the code there. You'll likely want to just copy an existing page template from your Theme, take out the inner loop, and replace it with your video code.
Adding the links to your 'header.php' file would be considered bad practice. You would want to create a 'functions.php' file inside your Child theme and call the JS/CSS from there. You can also find this within the WordPress codex (they literally give examples you can copy/paste and change your file names).

Extracting the header and the footer of html page

How to extract the header and the footer from this page to insert it in another page?
I'm a bit confused because when I copy and paste the header div it never has the same structure and graphics in the new page? Am I missing something here?

Typically when you copy/paste the div you're getting the HTML, but not the CSS styling (unless the styling is in-line).
There is a Chrome extension called "CSS + HTML" that allows you to, in the developer console, generate a version of the div that has all CSS turned into in-line CSS, so that you can copy/paste a pretty accurate version.
(Caveats: I've had some issues with the extension, so I don't enable it except when I need it, and the HTML produced is a) awful, because it has lots of unnecessary inline CSS, and b) not always a precise match. But it's pretty good.)

Yes, you are missing something. The CSS, images, and links... They are using relative links. You would need to be sure to replace those links.
The images are linked relatively so unless you copy them local you will not have access to them.
You would also need the Style Sheets as they are linked relatively in the head.
Not that the links in some cases are to .php files. Unless you know the php running in the background you are going to lose that functionality too.