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HTML

About HTML Preprocessors

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

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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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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).

Add External Stylesheets/Pens

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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.

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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.

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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 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 

              
                <!--
  Checkbox input used as a toggle thanks to the :checked pseudo-class.
  The `id` attribute is necessary to make it possible to check it through the label.
-->
<input type="checkbox" class="toggle" id="toggle" />

<!--
  Label from which the `for` attribute should match the one from the checkbox toggle.
-->
<label for="toggle">Toggle content</label>

<!--
  Content meant to be collapsed/expanded depending on the state of the checkbox.
-->
<div class="toggled">
  <p>Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Vestibulum tortor quam, feugiat vitae, ultricies eget, tempor sit amet, ante. Donec eu libero sit amet quam egestas semper. Aenean ultricies mi vitae est. Mauris placerat eleifend leo.</p>  
</div>
!

CSS 

              
                /**
 * Elegant and accessible content toggle with CSS.
 *
 * 1. Hides the toggle, as it can be toggled through the label thanks to the id/for combo.
 * 2. Hides the toggled content when not following a checked toggled.
 *
 * Courtesy to H5BP for the visually hidden ruleset: https://github.com/h5bp/html5-boilerplate/blob/master/src/css/main.css#L119).
 */
.toggle, /* 1 */
.toggle:not(:checked) ~ .toggled /* 2 */ {
  border: 0;
  clip: rect(0 0 0 0);
  height: 1px;
  margin: -1px;
  overflow: hidden;
  padding: 0;
  position: absolute;
  width: 1px;
}

.toggle:focus ~ label {
  color: deeppink;
}





























/**
 Demo styles, not interesting.
 **/

body {
  max-width: 600px;
  margin: 2em auto;
  background: #FDFDFD;
  padding: 1em;
}

label {
  border: 1px solid;
  padding: 5px;
  cursor: pointer;
}
!

JS
!
999px

Console