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HTML

              
                <header class="header">
</header>
  <div class="header-inner">
  <h1 class="header-title">Intersection Observer</h1>
    <h2 class="header-subtitle">The <em>intersectionRatio</em> retrieved from the Intersection Observer sets CSS vars to create the scroll effect.</h2>
  </div>
<main class="main">
  <div class="main-content toggle-class">
    <div class="main-content-side">
      <span>Aenean finibus dapibus</span>
      <img class="toggle-class" src="https://image.ibb.co/nRnqPU/adventure-altitude-austria-1008180-1539536183-85-171-133-3.jpg"/>
    </div>
    <div class="main-content-text">
      <p class="main-content-text-callout">After checking if an entry <em>isIntersecting</em>, it's possible to toggle the removal of a class to animate the entry of an element into the viewport on scroll. To prevent the animation from being triggered again, it's necessary to <em>unobserve</em> the entry.</p>
       <div class="main-content-text-col">
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut luctus at arcu ultrices sollicitudin. Aenean finibus dapibus mollis. Nam eget vehicula urna. Quisque rutrum suscipit massa, sit amet sollicitudin nisi feugiat quis. Suspendisse lorem magna, venenatis ut rutrum vitae, iaculis at lectus. Donec bibendum odio ac enim mattis, eget volutpat tortor malesuada. Aenean viverra molestie turpis, at tempus ligula fermentum ac. Nunc consequat, dui sed dapibus convallis, nisi enim tempor sem, finibus dignissim nunc ex ut urna. Ut a erat at tortor sodales sagittis id eget est. Nam id ornare elit. Mauris interdum in mi non consequat. Ut in sapien a velit malesuada gravida in quis augue. Integer luctus ut nunc sed tristique. Mauris diam quam, bibendum id purus a, consequat pulvinar nisi. Morbi odio diam, ullamcorper ut erat ullamcorper, blandit facilisis mi. Maecenas a nulla vel erat commodo congue.</p>
      </div>
    </div>
  </div>
</main>
              
            
!

CSS

              
                :root {
  --translate-value: -40;
}

html {
  box-sizing: border-box;
  cursor: default;
}

*,
*:before,
*:after {
  box-sizing: inherit;
}

body {
  background-color: #f6f8f9;
  color: #3d3d3d;
}

em {
  font-style: italic;
}

.header {
  height: 200%;
  background-image: url("https://i.postimg.cc/MTfZFb4d/img-1-compressor.jpg");
  background-size: cover;
  background-position: center center;
  background-color: #0a5ea2;
  background-blend-mode: multiply;
  position: fixed;
  width: 100%;
  top: 0;
  transform: translateY(calc(var(--translate-value) * 1%));
  will-change: auto;
  transition: 0.5s cubic-bezier(0.22, 0.44, 0, 1);
  &-inner {
    position: fixed;
    color: #fff;
    width: 100%;
    height: 100%;
    top: 0;
    display: flex;
    align-items: center;
    flex-direction: column;
    justify-content: center;
    transform: translateY(calc(var(--translate-value-header) * 1%));
    will-change: auto;
    transition: 0.8s cubic-bezier(0.22, 0.44, 0, 1);
  }
  &-title {
    text-align: center;
    font-family: "Roboto Condensed";
    text-transform: uppercase;
    font-weight: 900;
    font-size: 6vw;
    margin-bottom: 30px;
    letter-spacing: 0.2rem;
  }
  &-subtitle {
    text-align: center;
    font-family: "Playfair Display";
    font-weight: 400;
    -webkit-text-stroke: 0.015rem;
    letter-spacing: 0.03rem;
    font-size: 1.5rem;
  }
}

.main {
  margin-top: 101vh;
  padding: 120px 200px;
  background-color: #fefff4;
  position: relative;
  z-index: 1;
  &-content {
    display: flex;
    max-width: 1400px;
    margin: 0 auto;
    transition: 1.5s cubic-bezier(0.22, 0.44, 0, 1);
    &-side {
      flex-grow: 1;
      flex-basis: 0;
      text-transform: uppercase;
      font-family: "Roboto Condensed";
      font-weight: 700;
      color: #0a5ea2;
      letter-spacing: 0.15rem;
      font-size: 0.92rem;
      margin-top: 12px;
      margin-right: 40px;
      img {
        max-width: 100%;
        margin-top: 40px;
        transition: 1.5s cubic-bezier(0.22, 0.44, 0, 1) .4s;
      }
      &:before {
        content: "";
        height: 2px;
        width: 30px;
        background-color: #0a5ea2;
        display: inline-block;
        vertical-align: middle;
        transform: translateY(-2px);
        margin-right: 10px;
      }
    }
    &-text {
      flex-grow: 2;
      flex-basis: 0;
      font-family: "Playfair Display";
      line-height: 1.5;
      -webkit-text-stroke: 0.015rem;
      font-size: 1.25rem;
      font-weight: 400;
      margin-left: 40px;
      &-callout {
        font-weight: 700;
        margin-bottom: 60px;
        font-size: 1.5rem;
      }
      &-col {
        column-count: 2;
        column-gap: 40px;
      }
    }
  }
}

.toggle-class {
  opacity: 0;
  transform: translateY(120px);
}
              
            
!

JS

              
                const docStyle = document.documentElement.style;
const threshold = [...Array(100).keys()].map(i => i / 100);

const itemsObserver = new IntersectionObserver(
  entries => {
    entries.forEach(entry => {
      if (entry.isIntersecting) {
        document.documentElement.style.setProperty(
          "--translate-value",
          Math.round(-(1 - entry.intersectionRatio) * 40)
        );
        document.documentElement.style.setProperty(
          "--translate-value-header",
          Math.round(-(entry.intersectionRatio * 90))
        );
      }
    });
  },
  { threshold: threshold }
);

const itemsObserverSecondary = new IntersectionObserver(
  entries => {
    entries.forEach(entry => {
      if (entry.isIntersecting && entry.intersectionRatio > 0) {
        entry.target.classList.remove("toggle-class");
        itemsObserverSecondary.unobserve(entry.target);
      }
    });
  },
  { threshold: threshold }
);

itemsObserver.observe(document.querySelector(".main"));
const toggles = [...document.querySelectorAll(".toggle-class")];
toggles.forEach(toggle => {
  itemsObserverSecondary.observe(toggle);
})

              
            
!
999px

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