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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

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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Insecure Resource

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CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

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.

Vendor Prefixing

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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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

Add External Scripts/Pens

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

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You can apply a script from anywhere on the web to your Pen. Just put a URL to it here and we'll add it, in the order you have them, before the JavaScript in the Pen itself.

If the script you link to has the file extension of a preprocessor, we'll attempt to process it before applying.

You can also link to another Pen here, and we'll pull the JavaScript from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

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Packages

Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.

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About Packages

Using packages here is powered by Skypack, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ES6 import usage.

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HTML 

              
                <svg xmlns="http://www.w3.org/2000/svg" width="50" height="50" viewBox="0 0 50 50">
  <path fill="currentColor" d="M36.8861123,6.55651093e-05 C44.1405671,6.55651093e-05 49.9999344,9.15183928 49.9999344,16.964329 C49.9999344,32.3660946 24.9999672,50 24.9999672,50 L24.2999681,49.5044091 C20.2499734,46.585942 0,31.2779263 0,16.964329 C0,9.15183928 5.85936732,6.55651093e-05 13.1138221,6.55651093e-05 C18.3593509,6.55651093e-05 22.8794343,3.23666846 24.9999672,8.03576931 C27.1205002,3.23666846 31.6405835,6.55651093e-05 36.8861123,6.55651093e-05 Z"/>
</svg>

<svg xmlns="http://www.w3.org/2000/svg" width="50" height="50" viewBox="0 0 50 50">
  <path fill="currentColor" d="M9.09597022,27.2321727 L23.7722903,38.5044794 L49.9999344,5.80362938 L42.8012832,6.55651093e-05 L22.2097923,25.6138713 L14.7321235,19.8661109 L9.09597022,27.2321727 Z M35.7142389,44.6428642 L35.7142389,32.1428806 L41.0713747,32.1428806 L41.0713747,50 L0,50 L0,8.92862529 L17.8571194,8.92862529 L17.8571194,14.2857611 L5.35713583,14.2857611 L5.35713583,44.6428642 L35.7142389,44.6428642 Z"/>
</svg>

<svg xmlns="http://www.w3.org/2000/svg" width="50" height="50" viewBox="0 0 50 50">
  <path fill="currentColor" d="M41.0713747,8.92862529 C40.3459292,8.92862529 39.6762873,9.04023228 39.0066453,9.20764278 C37.8905753,3.96211394 33.258885,6.55651093e-05 27.6785351,6.55651093e-05 C21.2611328,6.55651093e-05 16.0714075,5.1897909 16.0714075,11.6071932 L16.0714075,12.5000492 L13.3928396,12.5000492 L13.3928396,11.6071932 C13.3928396,9.65407076 13.7834641,7.75675182 14.5089095,6.08264687 C13.3370361,5.63621889 12.0535556,5.3572014 10.7142717,5.3572014 C4.79910085,5.3572014 0,10.1563022 0,16.0714731 C0,21.9866439 4.79910085,26.7857447 10.7142717,26.7857447 L41.0713747,26.7857447 C45.9820826,26.7857447 49.9999344,22.7678929 49.9999344,17.857185 C49.9999344,12.9464772 45.9820826,8.92862529 41.0713747,8.92862529 Z M32.142815,30.3571686 L32.142815,33.9285925 L41.0713747,33.9285925 L26.7856792,50 L12.4999836,33.9285925 L21.4285433,33.9285925 L21.4285433,30.3571686 L32.142815,30.3571686 Z"/>
</svg>

<svg xmlns="http://www.w3.org/2000/svg" width="50" height="50" viewBox="0 0 50 50">
  <path fill="currentColor" d="M41.0713747,8.92862529 C40.3459292,8.92862529 39.6762873,9.04023228 39.0066453,9.20764278 C37.8905753,3.96211394 33.258885,6.55651093e-05 27.6785351,6.55651093e-05 C21.2611328,6.55651093e-05 16.0714075,5.1897909 16.0714075,11.6071932 L16.0714075,12.5000492 L13.3928396,12.5000492 L13.3928396,11.6071932 C13.3928396,9.65407076 13.7834641,7.75675182 14.5089095,6.08264687 C13.3370361,5.63621889 12.0535556,5.3572014 10.7142717,5.3572014 C4.79910085,5.3572014 0,10.1563022 0,16.0714731 C0,21.9866439 4.79910085,26.7857447 10.7142717,26.7857447 L41.0713747,26.7857447 C45.9820826,26.7857447 49.9999344,22.7678929 49.9999344,17.857185 C49.9999344,12.9464772 45.9820826,8.92862529 41.0713747,8.92862529 Z M12.4999836,44.6428642 L26.7856792,30.3571686 L41.0713747,44.6428642 L32.142815,44.6428642 L32.142815,50 L21.4285433,50 L21.4285433,44.6428642 L12.4999836,44.6428642 Z"/>
</svg>

<br>
<br>

<button>Change Color</button>
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CSS 

              
                body {
  text-align: center;
  padding: 50px;
  color: black;
}
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JS 

              
                const color = () => Math.floor(Math.random() * 255);

document.querySelector("button").onclick = () => {
  const [r, g, b] = [color(), color(), color()];
  document.body.style.color = `rgba(${r}, ${g}, ${b}, 1)`
};
!
999px

Console