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HTML 

              
                <header data-animation="fadeIn">
  <h1>
    <div class="title" data-animation="slideInDown" data-animation-delay=".75s"><b>Animate</b></div>
    <div class="subtitle" data-animation="slideInUp" data-animation-delay="1.5s">On Scroll</div>
  </h1>
</header>

<section>
  <h1><b>Slide</b> Animation</h1>
  <div class="box" data-animation="slideInRight"></div>
  <div class="box" data-animation="slideInUp" data-animation-delay=".3s"></div>
  <div class="box" data-animation="slideInDown" data-animation-delay=".6s"></div>
  <div class="box" data-animation="slideInLeft" data-animation-delay=".9s"></div>
</section>

<section>
  <h1><b>Zoom</b> Animation</h1>
  <div class="box" data-animation="zoomIn"></div>
  <div class="box" data-animation="zoomIn" data-animation-delay="300ms"></div>
  <div class="box" data-animation="zoomIn" data-animation-delay="600ms"></div>
</section>

<section>
  <h1><b>Reverse Zoom</b> Animation</h1>
  <div class="box" data-animation="zoomReverseIn"></div>
  <div class="box" data-animation="zoomReverseIn" data-animation-delay="300ms"></div>
  <div class="box" data-animation="zoomReverseIn" data-animation-delay="600ms"></div>
</section>

<section>
  <h1><b>Fade</b> Animation</h1>
  <div class="box" data-animation="fadeIn"></div>
  <div class="box" data-animation="fadeIn" data-animation-delay="300ms"></div>
  <div class="box" data-animation="fadeIn" data-animation-delay="600ms"></div>
</section>

<section>
  <h1><b>Flip</b> Animation</h1>
  <div class="box" data-animation="flipInY"></div>
  <div class="box" data-animation="flipInY" data-animation-delay="300ms"></div>
  <div class="box" data-animation="flipInY" data-animation-delay="600ms"></div>
</section>
!

CSS 

              
                // Variables
// ---------
:root {
  --animation-timing-function: cubic-bezier(0.4, 0, 0.2, 1);
  --animation-duration: 1s;
}

// Keyframes
// ---------
@keyframes slideInUp {
  0% {
    opacity: 0;
    transform: translateY(25%);
  }
  100% {
    opacity: 1;
    transform: none;
  }
}
@keyframes slideInDown {
  0% {
    opacity: 0;
    transform: translateY(-25%);
  }
  100% {
    opacity: 1;
    transform: none;
  }
}
@keyframes slideInleft {
  0% {
    opacity: 0;
    transform: translateX(25%);
  }
  100% {
    opacity: 1;
    transform: none;
  }
}
@keyframes slideInRight {
  0% {
    opacity: 0;
    transform: translateX(-25%);
  }
  100% {
    opacity: 1;
    transform: none;
  }
}

@keyframes fadeIn {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}

@keyframes zoomIn {
  0% {
    opacity: 0;
    transform: scale(0.75);
  }
  100% {
    opacity: 1;
    transform: none;
  }
}
@keyframes zoomReverseIn {
  0% {
    opacity: 0;
    transform: scale(1.25);
  }
  100% {
    opacity: 1;
    transform: none;
  }
}

@keyframes flipInY {
  0% {
    opacity: 0;
    transform: perspective(90vw) rotateY(67.50deg);
  }
  100% {
    opacity: 1;
    transform: none;
  }
}

// Animations
// ----------
[data-animation] {
  opacity: 0;
  animation-timing-function: var(--animation-timing-function);
  animation-fill-mode: both;
  animation-duration: var(--animation-duration);
  will-change: transform, opacity;
}

// Disable animation of the childs
// Disable self animation
.animations-disabled {
  &,
  [data-animation] {
    animation: none !important;
    opacity: 1 !important;
  }
}

// Slide Animations
.slideInUp {
  animation-name: slideInUp;
}
.slideInDown {
  animation-name: slideInDown;
}
.slideInLeft {
  animation-name: slideInleft;
}
.slideInRight {
  animation-name: slideInRight;
}

// Fade Animations
.fadeIn {
  animation-name: fadeIn;
}

// Zoom Animations
.zoomIn {
  animation-name: zoomIn;
}
.zoomReverseIn {
  animation-name: zoomReverseIn;
}

// Flip Animations
.flipInY {
  animation-name: flipInY;
}
.flipOutY {
  animation-name: flipInY;
  animation-direction: reverse;
}


// ===========
// Demo Styles
// ===========
* {
  box-sizing: border-box;
  line-height: calc(1em + 0.25rem);
}
html {
  background: #000;
  font: 18px/1.5 "Poppins", sans-serif;
  letter-spacing: -0.1em;
}
body {
  margin: 0;
  min-height: 100vh;
  overflow-x: hidden;
  color: hsl(210, 20%, 60%);
  font-weight: 200;
}
h1 {
  width: 100%;
  font-weight: 200;
  text-align: center;
  margin: 0 0 4rem;
  font-size: 3rem;
}
b {
  font-weight: 600;
}
header,
section {
  display: flex;
  flex-wrap: wrap;
  width: 100vw;
  height: 100vh;
  justify-content: space-evenly;
  align-items: center;
  align-content: center;
  padding: 3rem 1rem;
}
header {
  background: radial-gradient(
    circle at top right,
    hsl(340, 80%, 50%),
    hsl(260, 80%, 50%)
  );
  color: #fff;
}
header h1 {
  margin: 0;
  color: #fff;
}
header .title {
  font-size: 5rem;
}
header .subtitle {
  font-size: 4rem;
}
section {
  background: #fff;
  &:nth-child(odd) {
    background: hsl(210, 25%, 97.5%);
  }
}
.box {
  width: 7rem;
  height: 7rem;
  margin: 1rem;
  background: radial-gradient(
    circle at top right,
    hsl(340, 80%, 50%),
    hsl(260, 80%, 50%)
  );
  border-radius: 1.5rem;
  box-shadow: 0 0 0 3px rgba(255,255,255,0.3) inset, 0 3px 9px rgba(0, 0, 0, 0.3);
}
!

JS 

              
                const AnimateOnScroll = function ({ offset } = { offset: 10 }) {
  // Define a dobra superior, inferior e laterais da tela
  const windowTop = (offset * window.innerHeight) / 100;
  const windowBottom = window.innerHeight - windowTop;
  const windowLeft = 0;
  const windowRight = window.innerWidth;

  this.start = (element) => {
    window.requestAnimationFrame(() => {
      // Seta os atributos customizados
      element.style.animationDelay = element.dataset.animationDelay;
      element.style.animationDuration = element.dataset.animationDuration;

      // Inicia a animacao setando a classe para animar
      element.classList.add(element.dataset.animation);

      // Seta o elemento como animado
      element.dataset.animated = "true";
    });
  };

  this.inViewport = (element) => {
    // Obtem o boundingbox do elemento
    const elementRect = element.getBoundingClientRect();
    const elementTop =
      elementRect.top + parseInt(element.dataset.animationOffset) ||
      elementRect.top;
    const elementBottom =
      elementRect.bottom - parseInt(element.dataset.animationOffset) ||
      elementRect.bottom;
    const elementLeft = elementRect.left;
    const elementRight = elementRect.right;

    // Verifica se o elemento esta na tela
    return (
      elementTop <= windowBottom &&
      elementBottom >= windowTop &&
      elementLeft <= windowRight &&
      elementRight >= windowLeft
    );
  };

  // Percorre o array de elementos, verifica se o elemento está na tela e inicia animação
  this.verifyElementsInViewport = (els = elements) => {
    for (let i = 0, len = els.length; i < len; i++) {
      // Passa para o proximo laço se o elemento ja estiver animado
      if (els[i].dataset.animated) continue;

      this.inViewport(els[i]) && this.start(els[i]);
    }
  };

  // Obtem todos os elementos a serem animados
  this.getElements = () =>
    document.querySelectorAll("[data-animation]:not([data-animated])");

  // Atualiza a lista de elementos a serem animados
  this.update = () => {
    elements = this.getElements();
    elements && this.verifyElementsInViewport(elements);
  };

  // Inicia os eventos
  window.addEventListener("load", this.update, false);
  window.addEventListener(
    "scroll",
    () => this.verifyElementsInViewport(elements),
    { passive: true }
  );
  window.addEventListener(
    "resize",
    () => this.verifyElementsInViewport(elements),
    { passive: true }
  );
};

// Initialize
const options = {
  offset: 15 // percentage of the window
};

const animation = new AnimateOnScroll(options);
!
999px

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