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HTML

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Adding Classes

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About the <head>

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CSS

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CSS Base

About CSS Base

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <div class="site">
  <header>
    <nav>
      <ul role="list">
        <li>
          <a href="#first">First</a>
        </li>
        <li>
          <a href="#second">Second</a>
        </li>
        <li>
          <a href="#third">Third</a>
        </li>
        <li>
          <a href="#slider">Slider</a>
        </li>
        <li>
          <a href="#overflow">Overflow</a>
        </li>
      </ul>
    </nav>
  </header>

  <main>
    <section id="first">
      This is the first section. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Cum enim laborum aliquam quos beatae? Reprehenderit quibusdam sapiente mollitia nostrum sed consectetur, asperiores eveniet magnam quae quisquam quidem voluptas eligendi nam officiis harum vero maiores ab soluta iste. Id impedit, inventore dolor, totam esse eum, repudiandae dignissimos amet quo possimus ex. Lorem ipsum dolor sit amet consectetur adipisicing elit. Minus provident perspiciatis accusamus vel consequuntur doloribus, fugiat laudantium incidunt consequatur ipsam suscipit aliquid in culpa neque quae tenetur. Modi, labore autem!
    </section>
    <section id="second">
      This is the second section. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Cum enim laborum aliquam quos beatae? Reprehenderit quibusdam sapiente mollitia nostrum sed consectetur, asperiores eveniet magnam quae quisquam quidem voluptas eligendi nam officiis harum vero maiores ab soluta iste. Id impedit, inventore dolor, totam esse eum, repudiandae dignissimos amet quo possimus ex. Lorem ipsum dolor sit amet consectetur adipisicing elit. Excepturi possimus reiciendis quaerat, architecto culpa debitis molestiae numquam sequi deleniti qui corporis doloribus quibusdam ut pariatur magnam illo voluptatum vitae rerum natus aspernatur ab iste tempora! Corporis ipsa quam necessitatibus esse.
    </section>
    <section id="third">
      This is the third section. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Cum enim laborum aliquam quos beatae? Reprehenderit quibusdam sapiente mollitia nostrum sed consectetur, asperiores eveniet magnam quae quisquam quidem voluptas eligendi nam officiis harum vero maiores ab soluta iste. Id impedit, inventore dolor, totam esse eum, repudiandae dignissimos amet quo possimus ex. Lorem ipsum dolor sit amet, consectetur adipisicing elit. Fugit possimus assumenda quo praesentium dolorem esse asperiores rerum quod error quasi, sit ex accusantium architecto quaerat excepturi impedit repellat porro, culpa numquam est aut. Aut unde perferendis temporibus officiis possimus nobis reprehenderit rerum similique illum architecto?
    </section>

    <section id="slider">
      <figure>
        <img src='https://images.unsplash.com/photo-1631466266525-53e979c9e29b?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MnwxNDU4OXwwfDF8cmFuZG9tfHx8fHx8fHx8MTYzMjg1NDI4NQ&ixlib=rb-1.2.1&q=80&w=800' alt=''>
        <figcaption>
          <h1>This is a caption</h1>
        </figcaption>
      </figure>
      <figure>
        <img src='https://images.unsplash.com/photo-1630241040421-f777fdbc7c8e?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MnwxNDU4OXwwfDF8cmFuZG9tfHx8fHx8fHx8MTYzMjg1NDI3Nw&ixlib=rb-1.2.1&q=80&w=800' alt=''>
        <figcaption>
          <h1>This is a caption</h1>
        </figcaption>
      </figure>
      <figure>
        <img src='https://images.unsplash.com/photo-1631931020352-da7c5165e715?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MnwxNDU4OXwwfDF8cmFuZG9tfHx8fHx8fHx8MTYzMjg1NDI2Mg&ixlib=rb-1.2.1&q=80&w=800' alt=''>
        <figcaption>
          <h1>This is a caption</h1>
        </figcaption>
      </figure>
      <figure>
        <img src='https://images.unsplash.com/photo-1631168056607-dcb48cb3b63d?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MnwxNDU4OXwwfDF8cmFuZG9tfHx8fHx8fHx8MTYzMjg1NDI1NA&ixlib=rb-1.2.1&q=80&w=800' alt=''>
        <figcaption>
          <h1>This is a caption</h1>
        </figcaption>
      </figure>
    </section>
    
    <section id="overflow">
      <div class="overflow">
        Lorem ipsum dolor sit amet, consectetur adipisicing elit. Fugit maiores eius quidem labore magni temporibus fugiat repudiandae in ipsa voluptates aliquid debitis nesciunt sed itaque architecto, iste odio tenetur deserunt quam quas velit ipsum delectus accusantium. Natus totam tempora labore, eveniet consectetur veniam sequi illo numquam quas fugit, et in, explicabo itaque velit nesciunt adipisci ea omnis! Id debitis repellendus recusandae exercitationem aperiam ad ducimus. Reiciendis dolorem consequatur distinctio repudiandae voluptates, necessitatibus, esse autem consectetur quibusdam earum vero ipsa corporis. Lorem, ipsum dolor sit amet consectetur adipisicing elit. Sapiente quas dolores aspernatur debitis libero voluptatem quae, atque dicta ratione laborum quaerat! Nostrum eligendi facere totam doloribus dolore dolorum, distinctio veritatis quidem blanditiis iusto earum, aliquam nemo. Nisi eveniet facilis odit deserunt dolore, sapiente velit! Deserunt, odit quos doloribus eligendi suscipit ipsam repellendus voluptatem maiores deleniti obcaecati, consequuntur doloremque magni repellat neque nulla? Nulla dignissimos repellendus odit accusantium, deleniti beatae temporibus!
      </div>
    </section>

  </main>
</div>

<!--<div id="mark"></div>-->
!

CSS 

              
                html {
  --timing: .2s;
  scroll-behavior: smooth;
  scroll-padding-top: 60px;
}

body {
  //overscroll-behavior: none;
}

.site {
  display: grid;
  grid:
    auto /
    minmax(1em, 1fr)
    minmax(0, 600px)
    minmax(1em, 1fr);
  margin: 5rem auto;
}

main {
  padding-bottom: 80vh;
}

header {
  margin-bottom: 5rem;
  position: sticky;
  top: -1px;
  z-index: 10;
  box-shadow:
    0 20px 25px -5px rgba(0, 0, 0, 0.0001),
    0 10px 10px -5px rgba(0, 0, 0, 0.0001);
}

nav {
  background: #24b47e;
  overflow-x: auto;
  width: 100%;
  -webkit-overflow-scrolling: touch;
  @media (max-width: 420px) {
    background-image: 
	
    /* Shadows */ 
    linear-gradient(to right, #24b47e, #24b47e),
    linear-gradient(to right, #24b47e, #24b47e),
  
/* Shadow covers */ 
    linear-gradient(to right, rgba(0,0,0,.2), rgba(255,255,255,0)),
    linear-gradient(to left, rgba(0,0,0,.2), rgba(255,255,255,0));   

  background-position: left center, right center, left center, right center;
	background-repeat: no-repeat;
	background-color: #24b47e;
	background-size: 20px 100%, 20px 100%, 10px 100%, 10px 100%;
	
	/* Opera doesn't support this in the shorthand */
	background-attachment: local, local, scroll, scroll;
  }
}

nav::-webkit-scrollbar {
  display: none;
}

nav ul {
  display: flex;
  justify-content: space-evenly;
  text-align: center;
  flex-wrap: nowrap;
  @media (max-width: 420px) {
    justify-content: flex-start;
  }
}

nav a {
  color: #fff !important;
  padding: 1rem;
  display: inline-block;
  font-weight: 700;
}

section + section {
  margin-top: 3rem;
}


/* Slider */
#slider {
  width: 100%;
  overflow-x: auto;
  display: grid;
  grid: 400px / auto-flow 90%;
  gap: 10px;
  scroll-snap-type: x mandatory;
  overscroll-behavior-x: contain;
  padding-bottom: 1em;
  perspective: 2px;
  transform-style: preserve-3d;
}

figure {
  display: grid;
  place-items: end center;
  scroll-snap-align: center;
  scroll-snap-stop: always;
  overflow: hidden;
  background: #000;
  transform-style: preserve-3d;
}

img {
  height: 100%;
  width: 100%;
  object-fit: cover;
  transform: translateZ(-1px) scale(1.6);
  opacity: 0.5;
  will-change: transform;
}

img,
figcaption {
  grid-area: 1 / 1;
}

figcaption {
  padding: 2rem;
  z-index: 1;
}

h1 {
  font-size: 3rem;
  color: #fff;
}



/* Overflow */
.overflow {
  height: 300px;
  overflow: auto;
  overscroll-behavior: contain;
  padding: 2rem;
  background: #73e4a2;
  border-radius: 4px;
  box-shadow:
    0 20px 25px -5px rgba(0, 0, 0, 0.1),
    0 10px 10px -5px rgba(0, 0, 0, 0.04);
  color: #005450;
}


#mark {
  position: absolute;
  width: 1px;
  height: 1px;
  top: calc(var(--top) * 1px);
  left: 0;
}


html,
body {
  height: 100%;
  background: #f2f7f1;
}





/* Header effect */
.site > :not(header) {
  grid-column: 2 / -2;
}

header {
  display: grid;
  grid: inherit;
  grid-column: 1 / -1;
}

header > * {
  grid-column: 2 / -2;
}

header:before,
header:after {
  content: '';
  background: #24b47e;
  transform: scaleX(var(--resize, 0));
  transition: var(--timing) cubic-bezier(.5, 0, .5, 1);
  z-index: -1;
}
header:before {
  grid-column-start: 1;
  transform-origin: right;
}
header:after {
  grid-column-start: 3;
  transform-origin: left;
}

/*.resize-box header:before,
.resize-box header:after {
  --resize: 1;
}*/

.resize-box::before,
.resize-box::after {
  --resize: 1;
}

header {
  transition: calc(var(--timing) * .25) box-shadow;
}

/*.resize-box header {
  transition: calc(var(--timing) * 1.25) calc(var(--timing) * .25) box-shadow cubic-bezier(.5, 0, .5, 1);
  box-shadow: 0 20px 25px -5px rgba(0, 0, 0, 0.1), 0 10px 10px -5px rgba(0, 0, 0, 0.04);
}*/


.resize-box {
  transition: calc(var(--timing) * 1.25) calc(var(--timing) * .25) box-shadow cubic-bezier(.5, 0, .5, 1);
  box-shadow: 0 20px 25px -5px rgba(0, 0, 0, 0.1), 0 10px 10px -5px rgba(0, 0, 0, 0.04);
}
!

JS 

              
                //const bd = document.body;
const header = document.querySelector('header');
/*const headerTop = document.querySelector('header').getBoundingClientRect().top;

bd.style.setProperty('--top', headerTop)*/

/*let observer = new IntersectionObserver(entries => {
  !entries[0].isIntersecting ?
    bd.classList.add("resize-box") :
    bd.classList.remove("resize-box")
});*/

const observer = new IntersectionObserver( 
  ([e]) => e.target.classList.toggle("resize-box", e.intersectionRatio < 1),
  { threshold: 1 }
);

observer.observe(header);
!
999px

Console