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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

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

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

Any URLs added here will be added as <link>s in order, and before the CSS in the editor. You can use the CSS from another Pen by using its URL and the proper URL extension.

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About External Resources

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.

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

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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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About External Resources

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 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.

If you're using React / ReactDOM, make sure to turn on Babel for the JSX processing.

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HTML 

              
                <div id="top">
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Perferendis itaque veritatis et error hic, qui iste dolore soluta quos exercitationem fuga eveniet corporis labore natus eius quas tempora neque veniam?</div>
  <div>Et unde est quaerat, quam quia illo aut pariatur repudiandae cupiditate dolorum, amet corrupti, voluptates sint illum autem! Vel debitis quasi repellat tempore provident quod tempora, omnis minus consequuntur optio?</div>
  <div>Accusamus eum omnis, suscipit nobis id quis libero possimus laboriosam, neque expedita minus eaque molestias quasi quo cupiditate recusandae illum, tempore beatae alias est aliquid! Officia aut molestiae ducimus illum?</div>
  <div>Sapiente quia accusantium aspernatur assumenda perferendis, adipisci nisi odio velit hic corporis! Odio et neque ad veritatis maxime eveniet nobis odit tempore perferendis dolorum, explicabo distinctio nam, consequuntur exercitationem optio.</div>
  <div>Magni, rerum, consequuntur nihil aliquam eligendi ipsum voluptas laboriosam illum quae perspiciatis esse labore, dolores reiciendis! Veritatis porro non aperiam nisi voluptatem doloribus quaerat sed dignissimos! Eius accusamus ut facere!</div>
</div>
<div id="wrapper">
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Facere voluptate consequuntur consectetur temporibus facilis aspernatur ex nulla reiciendis aliquam corporis! Odit ab explicabo dicta rerum, numquam corporis repellendus incidunt. Reiciendis?</div>
  <div>Corrupti, impedit pariatur expedita excepturi suscipit voluptatum dolore eum sit amet culpa, quasi asperiores maiores explicabo perferendis cupiditate laborum quaerat sed quae voluptas delectus saepe reiciendis aliquam id iure. Placeat!</div>
  <div>Unde ea, temporibus enim reprehenderit recusandae facilis rerum dolore. Iste similique saepe, cupiditate minus inventore repellat nobis, voluptates nemo esse natus recusandae eum quisquam ipsa? Fuga sunt earum dicta repellendus?</div>
  <div>Accusamus, fugiat eligendi. Quia fugiat qui saepe quae porro. Omnis a inventore dicta quae soluta, voluptatem doloribus nobis obcaecati quo voluptate nulla quod reiciendis porro voluptates? Cum animi error modi.</div>
  <div>Quam officia commodi molestias itaque molestiae vitae tenetur, accusamus earum, ex expedita velit. A libero tempora quo quos aliquam iste assumenda ipsam consequatur, molestias eligendi vitae esse mollitia incidunt cumque!</div>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Laudantium neque a praesentium similique accusantium? Numquam minus vero nihil, modi in nulla voluptas saepe soluta esse dicta quis magnam unde repellat.</div>
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Debitis accusantium molestias odio sunt error. Vel tempora pariatur dicta dolor natus quia aliquam. Ullam doloremque soluta perferendis delectus ratione consequatur fugiat!</div><div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Quo possimus reiciendis sunt ut, voluptatum porro tenetur ullam! Harum assumenda, officia minima nam quia ex. Totam veniam eos assumenda culpa rerum.</div>
</div>
<div id="bottom">
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Sequi totam laborum, nam eius, id iusto enim vero doloremque perspiciatis architecto molestiae impedit aspernatur nobis vitae modi numquam harum cum provident.</div>
  <div>Perspiciatis cupiditate dicta ipsum accusamus, modi accusantium omnis molestias incidunt nostrum minima dolores dolorum porro aut quasi reiciendis, ut, at temporibus odio fugiat excepturi? Minima pariatur a aliquid asperiores velit!</div>
  <div>Autem unde accusantium eius nemo. Minus minima, quibusdam voluptatibus ducimus voluptate sit, aliquid qui hic soluta fugiat blanditiis quod unde corporis rem aliquam perferendis vitae pariatur facilis, officia et non.</div>
  <div>Obcaecati voluptates voluptate sequi at praesentium excepturi consequatur earum quos esse quas impedit delectus, id voluptatem possimus vero, explicabo eum velit aut, fugit vitae odit. Mollitia cum eos vero quae.</div>
  <div>Repudiandae voluptatibus aliquam ducimus earum. Optio labore laudantium sunt exercitationem quasi deleniti accusamus dolore nemo, natus ducimus non iste voluptates veritatis blanditiis vero fuga excepturi eaque. Ipsam delectus repudiandae cumque!</div>
  <div>Illum consequatur quibusdam veniam eos ea vero error, debitis voluptatum. Odio error iusto est eligendi non! Possimus, earum! Consequuntur eveniet ex illo voluptas aut ut neque, quam unde consequatur excepturi.</div>
  <div>Velit nemo adipisci possimus odio sed accusantium dolores voluptate autem repellendus delectus dicta fugiat mollitia, vel blanditiis laboriosam totam rerum voluptatum ut. Tempora pariatur expedita sint nemo impedit illum sequi.</div>
  <div>Expedita deleniti in, sit laborum adipisci sint alias deserunt tenetur, saepe fuga odio hic perspiciatis tempore doloremque, vero mollitia? Ab iste officia doloremque consequuntur perspiciatis libero quam expedita mollitia accusantium.</div>
</div>
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CSS 

              
                #wrapper{
  height :100px;
  overflow:auto;
  background: grey;
  padding: 10px;
}
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JS 

              
                window.addEventListener('scroll', function () {
  var vrednostWinScrolla = window.pageYOffset || document.documentElement.scrollTop || body.scrollTop || 0;
  console.log("Window scroll " + vrednostWinScrolla);
});

var el = document.getElementById("wrapper");
el.addEventListener('scroll', function () {
    console.log("Scroll unutrašnjeg elementa: " + el.scrollTop + "px");
});
!
999px

Console