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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

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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.

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.

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HTML 

              
                <div class="app-container">
  <header class="app-header">
    header
  </header>
  <div class="content">
    <p>
      Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque
      imperdiet ac est vitae convallis. Mauris sed neque ac metus pretium
      fermentum non sit amet lacus. Donec tristique erat at nunc blandit
      interdum. Vestibulum imperdiet libero ac sapien tempus ultricies. Cras et
      arcu ut nunc faucibus efficitur facilisis nec lorem. Duis ac placerat
      tortor. In nibh ex, tristique id malesuada vel, pharetra non turpis.
    </p>
    <p>
      Etiam tempus lorem dui, eget placerat massa blandit et. Aliquam erat
      volutpat. Mauris fringilla blandit congue. Pellentesque imperdiet mauris
      et eros tempor ultrices. Cras eu aliquet magna. Pellentesque at placerat
      ex. Morbi condimentum tincidunt urna, vel elementum lorem efficitur nec.
      Pellentesque venenatis feugiat nibh non malesuada. Ut id aliquet nulla.
      Aenean magna massa, ornare in diam sit amet, congue consequat tortor. Sed
      venenatis congue nisl, sed eleifend nibh molestie eu.
    </p>
    <p>
      Orci varius natoque penatibus et magnis dis parturient montes, nascetur
      ridiculus mus. Donec consequat odio sed neque semper, eget rutrum sapien
      ornare. Nunc eget varius tortor. Vestibulum iaculis diam a risus volutpat
      accumsan. Maecenas sit amet imperdiet leo. Aenean eleifend interdum
      pulvinar. Vestibulum lobortis ligula ut eros consectetur, a sollicitudin
      mi mattis. Donec gravida velit ut nisl mattis hendrerit. In at facilisis
      metus. Curabitur gravida ipsum eget urna lacinia euismod. Etiam in lorem
      facilisis enim tempus auctor. Curabitur eget massa risus. Ut elementum sem
      dictum tellus mollis pulvinar. Duis rhoncus turpis a magna dapibus
      pellentesque. Mauris bibendum eu nulla sit amet sodales. Fusce eget
      sodales purus.
    </p>
    <p>
      Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque
      imperdiet ac est vitae convallis. Mauris sed neque ac metus pretium
      fermentum non sit amet lacus. Donec tristique erat at nunc blandit
      interdum. Vestibulum imperdiet libero ac sapien tempus ultricies. Cras et
      arcu ut nunc faucibus efficitur facilisis nec lorem. Duis ac placerat
      tortor. In nibh ex, tristique id malesuada vel, pharetra non turpis.
    </p>
    <p>
      Etiam tempus lorem dui, eget placerat massa blandit et. Aliquam erat
      volutpat. Mauris fringilla blandit congue. Pellentesque imperdiet mauris
      et eros tempor ultrices. Cras eu aliquet magna. Pellentesque at placerat
      ex. Morbi condimentum tincidunt urna, vel elementum lorem efficitur nec.
      Pellentesque venenatis feugiat nibh non malesuada. Ut id aliquet nulla.
      Aenean magna massa, ornare in diam sit amet, congue consequat tortor. Sed
      venenatis congue nisl, sed eleifend nibh molestie eu.
    </p>
    <p>
      Orci varius natoque penatibus et magnis dis parturient montes, nascetur
      ridiculus mus. Donec consequat odio sed neque semper, eget rutrum sapien
      ornare. Nunc eget varius tortor. Vestibulum iaculis diam a risus volutpat
      accumsan. Maecenas sit amet imperdiet leo. Aenean eleifend interdum
      pulvinar. Vestibulum lobortis ligula ut eros consectetur, a sollicitudin
      mi mattis. Donec gravida velit ut nisl mattis hendrerit. In at facilisis
      metus. Curabitur gravida ipsum eget urna lacinia euismod. Etiam in lorem
      facilisis enim tempus auctor. Curabitur eget massa risus. Ut elementum sem
      dictum tellus mollis pulvinar. Duis rhoncus turpis a magna dapibus
      pellentesque. Mauris bibendum eu nulla sit amet sodales. Fusce eget
      sodales purus.
    </p>
    <p>
      Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque
      imperdiet ac est vitae convallis. Mauris sed neque ac metus pretium
      fermentum non sit amet lacus. Donec tristique erat at nunc blandit
      interdum. Vestibulum imperdiet libero ac sapien tempus ultricies. Cras et
      arcu ut nunc faucibus efficitur facilisis nec lorem. Duis ac placerat
      tortor. In nibh ex, tristique id malesuada vel, pharetra non turpis.
    </p>
    <p>
      Etiam tempus lorem dui, eget placerat massa blandit et. Aliquam erat
      volutpat. Mauris fringilla blandit congue. Pellentesque imperdiet mauris
      et eros tempor ultrices. Cras eu aliquet magna. Pellentesque at placerat
      ex. Morbi condimentum tincidunt urna, vel elementum lorem efficitur nec.
      Pellentesque venenatis feugiat nibh non malesuada. Ut id aliquet nulla.
      Aenean magna massa, ornare in diam sit amet, congue consequat tortor. Sed
      venenatis congue nisl, sed eleifend nibh molestie eu.
    </p>
    <p>
      Orci varius natoque penatibus et magnis dis parturient montes, nascetur
      ridiculus mus. Donec consequat odio sed neque semper, eget rutrum sapien
      ornare. Nunc eget varius tortor. Vestibulum iaculis diam a risus volutpat
      accumsan. Maecenas sit amet imperdiet leo. Aenean eleifend interdum
      pulvinar. Vestibulum lobortis ligula ut eros consectetur, a sollicitudin
      mi mattis. Donec gravida velit ut nisl mattis hendrerit. In at facilisis
      metus. Curabitur gravida ipsum eget urna lacinia euismod. Etiam in lorem
      facilisis enim tempus auctor. Curabitur eget massa risus. Ut elementum sem
      dictum tellus mollis pulvinar. Duis rhoncus turpis a magna dapibus
      pellentesque. Mauris bibendum eu nulla sit amet sodales. Fusce eget
      sodales purus.
    </p><p>
      Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque
      imperdiet ac est vitae convallis. Mauris sed neque ac metus pretium
      fermentum non sit amet lacus. Donec tristique erat at nunc blandit
      interdum. Vestibulum imperdiet libero ac sapien tempus ultricies. Cras et
      arcu ut nunc faucibus efficitur facilisis nec lorem. Duis ac placerat
      tortor. In nibh ex, tristique id malesuada vel, pharetra non turpis.
    </p>
    <p>
      Etiam tempus lorem dui, eget placerat massa blandit et. Aliquam erat
      volutpat. Mauris fringilla blandit congue. Pellentesque imperdiet mauris
      et eros tempor ultrices. Cras eu aliquet magna. Pellentesque at placerat
      ex. Morbi condimentum tincidunt urna, vel elementum lorem efficitur nec.
      Pellentesque venenatis feugiat nibh non malesuada. Ut id aliquet nulla.
      Aenean magna massa, ornare in diam sit amet, congue consequat tortor. Sed
      venenatis congue nisl, sed eleifend nibh molestie eu.
    </p>
    <p>
      Orci varius natoque penatibus et magnis dis parturient montes, nascetur
      ridiculus mus. Donec consequat odio sed neque semper, eget rutrum sapien
      ornare. Nunc eget varius tortor. Vestibulum iaculis diam a risus volutpat
      accumsan. Maecenas sit amet imperdiet leo. Aenean eleifend interdum
      pulvinar. Vestibulum lobortis ligula ut eros consectetur, a sollicitudin
      mi mattis. Donec gravida velit ut nisl mattis hendrerit. In at facilisis
      metus. Curabitur gravida ipsum eget urna lacinia euismod. Etiam in lorem
      facilisis enim tempus auctor. Curabitur eget massa risus. Ut elementum sem
      dictum tellus mollis pulvinar. Duis rhoncus turpis a magna dapibus
      pellentesque. Mauris bibendum eu nulla sit amet sodales. Fusce eget
      sodales purus.
    </p>
    <!-- another long content        -->
  </div>
</div>
!

CSS 

              
                
.app-container {
  margin: 0 auto;
  max-width: 750px;
}


.app-header {
/*   position: sticky;
  top: 0; */
  background-color: #282c34;
  min-height: 50px;
  max-width: 750px;
  width: 100%;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  font-size: calc(10px + 2vmin);
  color: white;
}

.sticky {
  position: fixed;
  top: 0;
}

.content {
  background: green;
  margin-top: 70px;
  color: white;
  padding: 10px;
}
!

JS 

              
                const header = document.querySelector('header');
var sticky = header.offsetTop;

window.addEventListener('scroll', () => {
  if (window.pageYOffset > sticky) {
    header.classList.add('sticky');
    
  }  else {
    header.classList.remove("sticky");
  }
});
!
999px

Console