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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).
Add External Stylesheets/Pens
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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>JS is fun :)</title>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<button onclick="openWindow()">Open the home page</button>
<button onclick="closeWindow()">Close the home page</button>
// CHANGE THE URL****************************************
myWindow = window.open("https://www.enjoytutorials.com", "myWindow", "width=200, height=100");