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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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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 

              
                <header>
  <h1>Website</h1>
</header>

<nav>
  <ul>
    <li><a href="#section-1">Section 1</a></li>
    <li><a href="#section-2">Section 2</a></li>
    <li><a href="#section-3">Section 3</a></li>
    <li><a href="#section-4">Section 4</a></li>
    <li><a href="#section-5">Section 5</a></li>
    <li><a href="#section-6">Section 6</a></li>
    <li><a href="#section-7">Section 7</a></li>
    <li><a href="#section-8">Section 8</a></li>
    <li><a href="#section-9">Section 9</a></li>
  </ul>
</nav>

<main>
  <section id="section-1">
    <h1>Section 1</h1>
    <p>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Officiis, blanditiis expedita? Earum eligendi pariatur quaerat quos expedita ab quibusdam ratione veniam in, mollitia fuga repudiandae?</p>
  </section>
  <section id="section-2">
    <h1>Section 2</h1>
    <p>Ratione nulla nam, ipsa dignissimos corrupti veniam nostrum, laudantium asperiores sequi numquam placeat velit voluptate in praesentium non labore unde incidunt laborum maxime quae magni.</p>
  </section>
  <section id="section-3">
    <h1>Section 3</h1>
    <p>Soluta quibusdam ad nostrum vel voluptate delectus sequi dolores quia quaerat officia corrupti, aperiam fugit facere debitis repudiandae praesentium sapiente inventore repellendus, nemo commodi alias!</p>
  </section>
  <section id="section-4">
    <h1>Section 4</h1>
    <p>Aliquid aliquam magnam ducimus similique obcaecati, unde exercitationem laborum incidunt, quas in ipsum inventore nostrum? Blanditiis optio cumque earum iste odio! Alias sint accusamus repudiandae.</p>
  </section>
  <section id="section-5">
    <h1>Section 5</h1>
    <p>Officia ipsum fugit iure eaque quisquam error tempore earum enim illum, delectus officiis incidunt corrupti aliquid nam quas perspiciatis eveniet doloremque quod labore? Doloremque, ipsum?</p>
  </section>
  <section id="section-6">
    <h1>Section 6</h1>
    <p>Aperiam repellat dignissimos fugiat possimus esse, suscipit neque nisi libero alias obcaecati ipsam, porro illo corrupti nostrum reprehenderit unde, illum in laudantium impedit. Modi, veniam.</p>
  </section>
  <section id="section-7">
    <h1>Section 7</h1>
    <p>Cum asperiores temporibus itaque consequatur quod inventore, quia quis explicabo dicta esse minus voluptatem reiciendis eveniet animi, necessitatibus illum dolorem doloremque repellat placeat, dolores eaque.</p>
  </section>
  <section id="section-8">
    <h1>Section 8</h1>
    <p>Optio qui, omnis itaque rerum iusto molestiae necessitatibus deleniti quod tenetur id perspiciatis voluptatum dolorum quisquam eius ipsum non architecto labore! Distinctio, tenetur. Officiis, necessitatibus?</p>
  </section>
  <section id="section-9">
    <h1>Section 9</h1>
    <p>Rem iste iure blanditiis excepturi esse nisi corrupti sequi, illo, laborum quo quis quaerat assumenda perspiciatis quod fuga vel laudantium doloribus architecto tempora omnis earum!</p>
  </section>
</main>

<footer>
  &copy;2018 Footer
</footer>
!

CSS 

              
                html {
  scroll-behavior: smooth;
}
body {
  margin: 0;
  display: grid;
  grid-template-columns: min-content 1fr;
  font-family: system-ui, sans-serif;
  
  /* this breaks position sticky in Firefox */
  /* overflow-x: hidden; */
}
header {
  grid-column: 1 / 3;
  background: #455A64;
  color: white;
  padding: 4rem;
  text-align: center;
}
nav {
  white-space: nowrap;
  background: #37474F;
}
nav ul {
  list-style: none;
  margin: 0;
  padding: 0;
}
/* Only stick if you can fit */
@media (min-height: 300px) {
  nav ul {
    position: sticky;
    top: 0;
  }
}
nav ul li a {
  display: block;
  padding: 0.5rem 1rem;
  color: white;
  text-decoration: none;
}
nav ul li a.current {
  background: black;
}
main {
  padding-bottom: 40rem;
}
section {
  padding: 2rem;
  margin: 0 0 2rem 0;
}
footer {
  grid-column: 1 / 3;
  background: #607D8B;
  padding: 5rem 1rem;
}
!

JS 

              
                let mainNavLinks = document.querySelectorAll("nav ul li a");
let mainSections = document.querySelectorAll("main section");

let lastId;
let cur = [];

// This should probably be throttled.
// Especially because it triggers during smooth scrolling.
// https://lodash.com/docs/4.17.10#throttle
// You could do like...
// window.addEventListener("scroll", () => {
//    _.throttle(doThatStuff, 100);
// });
// Only not doing it here to keep this Pen dependency-free.

window.addEventListener("scroll", event => {
  let fromTop = window.scrollY;

  mainNavLinks.forEach(link => {
    let section = document.querySelector(link.hash);

    if (
      section.offsetTop <= fromTop &&
      section.offsetTop + section.offsetHeight > fromTop
    ) {
      link.classList.add("current");
    } else {
      link.classList.remove("current");
    }
  });
});
!
999px

Console