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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.

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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.

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Packages

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

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

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HTML 

              
                <div id="progressBar"></div>
<div class="container">
  <h1>Scroll Down</h1>
  <div class="container-body">
    <p>
    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc placerat, purus ac mattis scelerisque, sapien justo efficitur eros, non consectetur nulla libero sed metus. Aenean a purus porttitor, volutpat velit vel, luctus mauris. Etiam ornare eros nec odio eleifend, eget maximus tortor aliquet. Phasellus at porttitor sem. Sed accumsan consectetur enim, nec pellentesque felis congue id. Vestibulum sed commodo urna, hendrerit vehicula tellus. Vestibulum imperdiet malesuada mattis. Interdum et malesuada fames ac ante ipsum primis in faucibus. Quisque in molestie mauris. Aliquam ligula libero, suscipit vitae luctus sed, posuere sit amet sem. Curabitur nulla leo, semper vitae massa in, consectetur fringilla odio.
    </p>
    Aenean ornare, dolor nec fermentum accumsan, nisi enim maximus quam, at volutpat nisi odio ut risus. Nam faucibus, sapien sit amet pellentesque euismod, purus nibh dapibus nulla, nec fermentum lectus tellus ac mauris. Nunc eu dolor eu nisl volutpat aliquet nec ut nisi. Morbi tristique libero non velit dapibus, nec facilisis enim molestie. Aliquam eu neque arcu. Ut dictum nibh interdum laoreet volutpat. Phasellus scelerisque mi in felis posuere feugiat. Mauris ut urna auctor, porta mauris sit amet, viverra nulla. Fusce vitae nulla placerat ipsum iaculis molestie eget luctus risus. Nulla ligula odio, tincidunt sed egestas sit amet, malesuada nec augue.
    </p>
  <p>
  Phasellus nec congue metus. Mauris commodo, justo sollicitudin euismod egestas, purus dui porttitor sem, at tincidunt nulla metus vitae velit. Nam faucibus, turpis nec blandit tempor, orci nisl efficitur leo, nec bibendum lectus mi eget justo. In pretium leo vitae ligula condimentum tristique. Pellentesque quis tristique est. Maecenas ac laoreet risus. Etiam in ante eu ligula molestie bibendum pretium vel sapien. Maecenas commodo faucibus dui, nec auctor libero lobortis eu. Maecenas quis ligula tincidunt, ultrices mi eget, mollis mi. Integer malesuada diam eu enim sodales, vel sagittis nunc porta. Suspendisse efficitur vulputate libero, id pellentesque nunc viverra sit amet. Donec placerat porta diam eget ullamcorper. Ut a elit porttitor elit gravida fringilla ut a nulla. Maecenas ut dui ullamcorper, maximus orci in, faucibus ligula. Etiam efficitur, odio sed tempus laoreet, neque felis accumsan dui, at commodo elit odio ut neque. Aliquam auctor quam erat, a sollicitudin lacus blandit at.

Suspendisse risus mauris, dignissim eget eleifend vitae, dignissim eget turpis. Duis id est non orci gravida sagittis. Aliquam non lacinia est. Ut consequat convallis felis vitae tincidunt. Aenean pharetra elit vel ante sollicitudin, sit amet rutrum urna auctor. Donec vitae enim quam. Proin eu consectetur sapien. Aenean euismod semper massa. Ut vehicula dapibus consequat. In non imperdiet leo, id ullamcorper felis. Integer metus nibh, rutrum ac iaculis lobortis, imperdiet sed est. Aenean vestibulum tempor elit ut blandit. Nam commodo rutrum eros pharetra ultricies.

Integer sit amet consequat magna. Sed sed tempus enim. Aliquam ultrices urna nec lorem pellentesque tempor. Nullam aliquam ut nisl et egestas. Curabitur urna lectus, faucibus sed interdum at, malesuada et ligula. Donec semper purus odio, in rutrum leo pellentesque ullamcorper. Mauris quis orci ipsum.
  </p>
  <p>
 Suspendisse eu tortor at ipsum lacinia venenatis in quis tortor. Morbi eget est eget neque vulputate tincidunt. Sed non elit mi. Aliquam tristique libero id tellus accumsan hendrerit. Pellentesque elementum augue risus, id eleifend metus faucibus sit amet. Mauris congue tempus blandit. Mauris nec metus et magna consectetur condimentum. Integer sit amet velit tellus. Maecenas sit amet mi arcu. Nunc egestas sollicitudin libero quis luctus. Pellentesque vitae sodales tortor. Mauris lectus sapien, facilisis vel placerat ut, imperdiet congue mauris.

Mauris tempus eros arcu, sit amet ullamcorper ligula consectetur vitae. Curabitur semper sollicitudin ornare. Morbi in dui tincidunt, vulputate odio sed, ornare leo. Proin sed faucibus quam. Mauris ornare enim vel dictum molestie. Curabitur a nulla nulla. Praesent dictum tellus id ipsum tempor commodo. 
  </p>
    <p>
    Duis faucibus, velit sit amet imperdiet interdum, libero augue dapibus nulla, sed congue orci diam eu dolor. Interdum et malesuada fames ac ante ipsum primis in faucibus. Proin justo velit, accumsan at euismod ac, sollicitudin fermentum leo. Proin ut rutrum felis, a placerat elit. Quisque id eleifend neque. Praesent non sodales odio. Aliquam molestie venenatis justo, quis efficitur mauris dictum eget. Donec lobortis luctus ex. Aenean non metus nisl. Mauris nec nunc arcu. Nam mollis ipsum ac mi fermentum dignissim. Donec justo odio, efficitur eu hendrerit volutpat, posuere a enim. Sed sit amet blandit tortor, vitae rhoncus nisi. Aliquam vehicula malesuada urna nec eleifend. Duis aliquam, lacus eu tincidunt malesuada, nunc libero tempor nisi, sed dictum justo magna eget risus. Sed consequat sodales leo vitae bibendum.
  </o>
  </div>
</div>
!

CSS 

              
                body {
  margin: 0;
  background: #EEEEEE;
}

#progressBar {
  position: fixed;
  top: 0;
  width: 0%;
  height: 8px;
  background: #F44336;
  transition: width 0.2s ease;
}

.container {
  max-width: 650px;
  margin: 40px auto;
  line-height: 16px;
  font-family: 'Open Sans', sans-serif;
  font-size: 1.125em;
}

.container-body {
  line-height: 1.6;
}
!

JS 

              
                const body = document.body;
const documentElement = document.documentElement;
const progressBarEl = document.getElementById('progressBar');

const scrollTop = () => Math.max(body.scrollTop, documentElement.scrollTop);
const handleOnScroll = () => {
  const scrollAmount = scrollTop() / (documentElement.scrollHeight - documentElement.clientHeight);
  const scrollPercentage = Math.round(scrollAmount * 100);
  
  progressBarEl.style.width = `${scrollPercentage}%`
};

window.addEventListener('scroll', handleOnScroll);
!
999px

Console