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HTML

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Adding Classes

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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

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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

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <header>
  Header
</header>

<main>
  <p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Distinctio adipisci iure quibusdam assumenda beatae repellendus similique commodi numquam laudantium, obcaecati voluptatem praesentium quod blanditiis neque animi velit dolorem itaque at?</p>
  <p>Rerum dignissimos placeat commodi exercitationem accusantium. Iusto quos sed delectus! Itaque, ad? Hic nesciunt doloribus quidem magni impedit quae tempora commodi ipsum nulla mollitia, dignissimos iusto. Quisquam officia voluptatem deserunt.</p>
  <p>Esse mollitia dolorem blanditiis quo omnis dolor? Eaque fugit velit voluptatibus quibusdam nihil et? Iusto asperiores sit, natus officiis sapiente tempora, doloribus odit unde, eaque voluptatum veritatis aspernatur quae provident?</p>
  <p>Beatae asperiores nostrum qui natus aliquid expedita animi ad omnis illum blanditiis maiores molestias odio repudiandae eos magni quia, quod deleniti perspiciatis porro vitae? Quisquam debitis in doloribus maiores illo?</p>
  <p>Tempora perferendis in exercitationem neque ipsum veritatis veniam molestiae odit omnis culpa. At neque nihil alias iure in dolore error eaque, accusantium est, assumenda, unde ullam ut cupiditate quidem officia!</p>
  <p>Libero veniam itaque, doloremque, soluta molestiae illo atque rerum eligendi facilis est labore eveniet possimus mollitia deleniti quaerat fuga at, harum similique dicta quos. Necessitatibus, quas sequi. Laboriosam, maxime eius.</p>
  <p>Optio minus placeat aperiam, accusantium doloribus ducimus mollitia architecto, earum explicabo labore rem voluptatem, error vero pariatur veritatis? Quidem impedit nobis doloribus possimus? Ex, nesciunt modi iure maxime fugiat provident!</p>
  <p>Est eum praesentium maiores cupiditate necessitatibus dicta tenetur perspiciatis porro culpa numquam voluptatibus quaerat itaque aperiam, repudiandae tempore asperiores velit delectus impedit? Facere eos nemo exercitationem necessitatibus inventore reprehenderit illo?</p>
  <p>Obcaecati ex recusandae sint, debitis enim laudantium harum repudiandae eveniet at minima voluptas nobis excepturi optio quam possimus, quod modi voluptatum necessitatibus impedit! Fugiat officia fugit quasi illo cupiditate consequuntur.</p>
  <p>Dolor corporis culpa ullam sapiente! Excepturi asperiores totam fuga incidunt ea, adipisci possimus dolor quas accusantium atque quos! Qui consectetur maiores earum deleniti pariatur minus quas sed adipisci ratione molestias?</p>
</main>

<div id="top-of-site-pixel-anchor"></div>
!

CSS 

              
                header {
  background: purple;
  padding: 1rem;
  color: white;
  
  body.header-not-at-top & {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
  }
  @at-root {
    body.header-not-at-top {
      padding-top: 44px;
    }
  }
  
}

#top-of-site-pixel-anchor {
  position: absolute;
  width: 1px;
  height: 1px;
  top: 500px;
  left: 0;
}

body {
  font-family: system-ui;
  line-height: 1.4;
  font-size: 125%;
  margin: 0;
  padding: 10px;
}

main {
  max-width: 500px;
  margin: 20px auto;
}
!

JS 

              
                if (
  "IntersectionObserver" in window &&
  "IntersectionObserverEntry" in window &&
  "intersectionRatio" in window.IntersectionObserverEntry.prototype
) {
let observer = new IntersectionObserver(entries => {
  if (entries[0].boundingClientRect.y < 0) {
    document.body.classList.add("header-not-at-top");
  } else {
    document.body.classList.remove("header-not-at-top");
  }
});
observer.observe(document.querySelector("#top-of-site-pixel-anchor"));
}
!
999px

Console