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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

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About the <head>

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Insecure Resource

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CSS

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CSS Base

About CSS Base

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Vendor Prefixing

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <section class="fs-l-513">
  <div class="container">
    <div class="content">
      <div class="wrapper-sticky">
        <strong>
          Example
        </strong>
        <h2>
          Youtube Shorts
        </h2>
        <p>
          Quick hits from one of the friendliest folks in CSS.
        </p>

        <ul class="images-content" id="gsapImagesContent">
          <li>
            <h3>
              Underused CSS Features
            </h3>
            <p>
              overscroll-behavior: contain is super useful, has great browser support, and yet a lot of people aren't even aware of it!
            </p>
            <iframe class="hide-at-full" width="315" height="560"
src="https://www.youtube.com/embed/FbV4M_ynSi0"
title="YouTube video player"
frameborder="0"
allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
allowfullscreen></iframe>
          </li>
          <li>
            <h3>
              Scroll-based animations
            </h3>
            <p>
              Scroll-based animations with CSS are incredibly powerful AND super easy to use, can't ask for much more than that!
            </p>
            <iframe class="hide-at-full" width="315" height="560"
src="https://www.youtube.com/embed/5DfWwzzsuoE"
title="YouTube video player"
frameborder="0"
allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
allowfullscreen></iframe>
          </li>
          <li>
            <h3>
              Daily Dev
            </h3>
            <p>
              Sponsor for Kevin, but just a helpful resource to me. Check out daily dev to keep up with CSS, and all topics front-end related! <a href="https://daily.dev/kevin-powell">https://daily.dev/kevin-powell</a>
            </p>
            <iframe class="hide-at-full" width="315" height="560"
src="https://www.youtube.com/embed/45PE03Ca1Lg"
title="YouTube video player"
frameborder="0"
allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
allowfullscreen></iframe>
          </li>
          <li>
            <h3>
              New ways to animate display
            </h3>
            <p>
              We used to have to come up with workarounds for animating and transitioning to and from display: none, but it's really easy to do now!
            </p>
            <iframe class="hide-at-full" width="315" height="560"
src="https://www.youtube.com/embed/owFsdm8q7rM"
title="YouTube video player"
frameborder="0"
allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
allowfullscreen></iframe>
          </li>
        </ul>
      </div>
    </div>
    <div class="show-at-full">
      <ul class="images" id="gsapImages">
        <li>
          <iframe width="315" height="560"
src="https://www.youtube.com/embed/FbV4M_ynSi0"
title="YouTube video player"
frameborder="0"
allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
allowfullscreen></iframe>
        </li>
        <li>
          <iframe width="315" height="560"
src="https://www.youtube.com/embed/5DfWwzzsuoE"
title="YouTube video player"
frameborder="0"
allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
allowfullscreen></iframe>
        </li>
        <li>
          <iframe width="315" height="560"
src="https://www.youtube.com/embed/45PE03Ca1Lg"
title="YouTube video player"
frameborder="0"
allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
allowfullscreen></iframe>
        </li>
        <li>
          <iframe width="315" height="560"
src="https://www.youtube.com/embed/owFsdm8q7rM"
title="YouTube video player"
frameborder="0"
allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
allowfullscreen></iframe>
        </li>
      </ul>
    </div>
  </div>
</section>
!

CSS 

              
                .content {
  margin-inline: auto;
  max-width: 600px;
  padding-block: 2rem;
}

.fs-l-513 {
  padding-inline: 1rem;
}

.images-content {
  margin-block-start: 4rem;
}

.show-at-full {
  display: none;
}

@media screen and (min-width: 768px) {
  .container {
    display: grid;
    gap: 2rem;
    grid-template-columns: 1fr 1fr;
    max-inline-size: 1280px;
    margin-inline: auto;
  }
  
  .content {
    margin-inline: 0;
    max-inline-size: auto;
    padding-block-start: 4rem;
  }
  
  .hide-at-full {
    display: none;
  }
  
  .images {
    padding-bottom: 4rem;
  }
  
  .images li img {
    margin-block-start: 5rem;
  }
  
  .images-content {
    display: grid;
  }
  
  .images-content li {
    grid-column-start: 1;
    grid-row-start: 1;
  }
  
  .images-content li:not(:first-child) {
    opacity: 0;
    pointer-events: none; 
  }
  
  .show-at-full {
    display: block;
  }
  
  .wrapper-sticky {
    position: sticky;
    top: 4rem;
  }
}

/* Content Styles */

@import url('https://fonts.googleapis.com/css2?family=Roboto:wght@400;700&display=swap');

:root {
  --color-bg: #fff;
  --color-text: #000;
  --color-heading: #000;
}

a {
  color: #ff0000;
}

a:active, a:focus, a:hover {
  opacity: 0.5;
}

body {
  background: var(--color-bg);
  color: var(--color-text);
  font-family: "Roboto", sans-serif;
  font-weight: 400;
  font-style: normal;
  font-size: clamp(1rem, 0.925rem + 0.32vw, 1.125rem);
  line-height: 1.5;
  margin: 0;
}

pre {
  background-color: var(--color-bg-code);
  box-shadow: var(--shadow);
  padding: 1rem;
  white-space: normal;
}

h2, h3 {
  color: var(--color-heading);
}

h2, h3, strong {
  font-weight: 700;
}

h2 {
  font-size: clamp(2.25rem, 1.8rem + 1.92vw, 3rem);
  line-height: 1.25;
  margin-block: 0.5rem;
}

h3 {
  font-size: clamp(1.25rem, 1.1rem + 0.64vw, 1.5rem);
  margin-block-end: 0;
  margin-block-start: 2rem;
}

img {
  box-shadow: var(--shadow);
}

iframe {
  margin-block-start: 2rem;
}

iframe {
  display: block;
  margin-inline: auto;
}

.link-button {
  align-items: center;
  border: 1px solid #000;
  display: inline-flex;
  color: #000;  
  justify-content: center;
  padding-inline: 1rem;
  padding-block: 0.5rem;
  text-decoration: none;
}

@media screen and (min-width: 768px) {
  h3 {
    margin-block-start: 0;
  }
}

/* Reset Styles */

* {
  box-sizing: border-box;
}

img {
  display: block;
  height: auto;
  max-inline-size: 100%;
}

ul, li {
  list-style: none;
  margin: 0;
  padding: 0;
}
!

JS 

              
                gsap.registerPlugin(ScrollTrigger);

const imagesContentItems = document.querySelectorAll("#gsapImagesContent li");
const imagesItems = document.querySelectorAll("#gsapImages li");

gsap.matchMedia().add("(min-width: 768px)", function() {
  imagesItems.forEach((image, index) => {
    ScrollTrigger.create({
      trigger: image,
      start: "top center",
      end: "bottom center",
      onEnter: () => {
        gsap.to(imagesContentItems[index], { opacity: 1, y: 0, pointerEvents: "auto", duration: 0.5 });
      },
      onLeave: () => {
        gsap.to(imagesContentItems[index], { opacity: 0, y: -48, pointerEvents: "none", duration: 0.5 });
      },
      onEnterBack: () => {
        gsap.to(imagesContentItems[index], { opacity: 1, y: 0, pointerEvents: "auto", duration: 0.5 });
      },
      onLeaveBack: () => {
        gsap.to(imagesContentItems[index], { opacity: 0, y: 48, pointerEvents: "none", duration: 0.5 });
      },
    });
  });
});
!
999px

Console