In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.
About the <head>
In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.
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About CSS Preprocessors
CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.
It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.
About Vendor Prefixing
To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.
We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).
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You can apply CSS to your Pen from any stylesheet on the web. Just put a URL to it here and we'll apply it, in the order you have them, before the CSS in the Pen itself.
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If the script you link to has the file extension of a preprocessor, we'll attempt to process it before applying.
<title> Learning HTML is fun :) </title>
<video controls preload="none" loop width="700px" height="300px">
<source src="https://file-examples.com/wp-content/uploads/2017/04/file_example_MP4_480_1_5MG.file_example_MP4_480_1_5MG" type="video/mp4">
<source src="https://file-examples.com/wp-content/uploads/2018/04/file_example_OGG_480_1_7mg.ogg" type="video/ogg">
This browser does not support HTML5 video player!