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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>

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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).

Add External Stylesheets/Pens

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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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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

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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.

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

All packages are different, so refer to their docs for how they work.

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HTML 

              
                <!--
Example of triggering a CSS animation from JavaScript. Modified from https://codepen.io/Orb89/pen/zvOEWM
-->

<div class="wrapper">
		  <div id="red-box">
        <span id="click-text">Click Me</span>
  </div>
</div>
!

CSS 

              
                /* Set up our red box */
#red-box {
  height: 100px;
  width: 100px;
  background: red;
  margin: 0 auto;
  display: table;
  text-align: center;
  color: white;
}

/* Define the frames our animation and name the animation boxmorph */
@keyframes boxmorph {
  40%{
    height: 30px;
    width: 30px;
    background: green;
    border-radius: 50px;
  }
}

/* Adding this class will trigger the animation */
.animate {
  animation: boxmorph 5s;
}

/* Center text */
#click-text {
  display: table-cell;
  vertical-align: middle;
}

#wrapper{
  position: relative;  
}
!

JS 

              
                /* Get the red box */
box_div = document.getElementById('red-box');

/* When the box is clicked, trigger the animation by adding the .animate class we defined in the CSS */
box_div.addEventListener('click', () => {
  console.log('Starting animation');
  box_div.classList.add('animate');
});

/* When the animation has ended, remove the .animate class so we can add it again when the box is clicked the next time (otherwise we can't retrigger the animation) */
box_div.addEventListener('animationend', () => {
  console.log('Animation endeded');
  box_div.classList.remove('animate');
});
!
999px

Console