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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

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.

!
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.

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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 

              
                <div class="main id">
  <div id="progress"></div>
  <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Nam cumque, reiciendis dolore, aperiam laudantium consectetur, doloribus quam earum repellat dicta ipsum? Magnam quaerat iste exercitationem eius esse est excepturi alias?</p>
  <p>Doloremque nulla consequuntur dolorem reiciendis, provident iure ipsa totam consectetur, quos illo corporis voluptate. Neque ullam eum corporis sed. Adipisci animi hic ipsam dolore eos asperiores voluptas nulla officia obcaecati.</p>
  <p>Incidunt sed, consectetur velit quod dolorum qui voluptas voluptate soluta? Itaque magnam eum sed, temporibus nobis voluptate nemo quam beatae iste id, debitis possimus. Minima ad blanditiis omnis nesciunt animi!</p>
  <p>Officiis adipisci reprehenderit culpa similique cum unde cumque ducimus, voluptate ipsam omnis laboriosam. Facilis doloribus pariatur neque repudiandae, incidunt distinctio maiores eius a, itaque ea modi accusantium! Obcaecati, explicabo vitae!</p>
  <p>Deserunt excepturi placeat reiciendis eaque iure odit numquam, voluptatibus amet ea soluta tenetur quibusdam asperiores, animi eligendi unde facere dolore tempora delectus praesentium! Obcaecati, non rerum! Porro perferendis cumque quod!</p>
  <p>Veniam aliquam laboriosam maxime beatae nam molestiae facilis libero, ullam blanditiis magnam, reiciendis voluptas modi, illo deserunt sint tempore dolor voluptatum vel quibusdam molestias? Fugiat vitae eligendi iste amet dolores!</p>
  <p>Quisquam, ipsum dolore. Reiciendis sequi doloremque, qui officiis id fuga voluptatem tempore temporibus quos delectus ea eum expedita earum nihil quas, numquam vero quisquam aliquam provident optio commodi explicabo ab.</p>
  <p>Molestias sunt, ea quod optio commodi maxime aut suscipit, aspernatur ullam quia recusandae fugiat distinctio ipsam ipsa! Cum illo ullam numquam quas quos, laboriosam veniam repudiandae tempore quaerat deleniti eius.</p>
  <p>Tenetur autem, aut repellendus asperiores sed quis nihil deleniti dolor rem, vel id architecto cum repudiandae ea debitis numquam nesciunt suscipit minus laudantium voluptate. Totam ipsum amet deleniti nam optio?</p>
  <p>Exercitationem id corrupti minima tempora deleniti dolore, suscipit sit laboriosam voluptatum, vitae deserunt illo assumenda necessitatibus cum reiciendis sint facere et tempore quam totam odio molestias aspernatur quisquam. Veritatis, minima?</p>
</div>
!

CSS 

              
                *,
::after,
::before {
  box-sizing: border-box;
}

body {
  font-family: system-ui, sans-serif;
  font-size: 1.25em;
}

.main {
  line-height: 1.4;
  inline-size: 80vw;
  margin: 0 auto;

  p {
    inline-size: 80ch;
  }
}

#progress {
  position: fixed;
  left: 0;
  top: 0;
  width: 100%;
  height: 0.5rem;
  background: red;
}
!

JS 

              
                const $progressbar = document.querySelector("#progress");

$progressbar.style.transformOrigin = "0% 50%";
$progressbar.animate(
  {
    transform: ["scaleX(0)", "scaleX(1)"]
  },
  {
    fill: "forwards",
    timeline: new ScrollTimeline({
      source: document.documentElement
    })
  }
);
!
999px

Console