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HTML 

              
                <div class="box-wrapper">
    <div class="box js-scroll-fall">
      <p class="text">スクロール↓</p>
      <div class="item"></div>
      <div class="item item--color2"></div>
      <div class="item"></div>
      <div class="item item--horizontal"></div>
      <div class="item item--color2"></div>
      <div class="item"></div>
      <div class="item item--color3"></div>
      <div class="item"></div>
      <div class="item item--color3 item--horizontal"></div>
      <div class="item item--color2"></div>
      <div class="item"></div>
      <div class="item item--color3 item--horizontal"></div>
      <div class="item"></div>
      <div class="item item--color2"></div>
      <div class="item item--horizontal"></div>
    </div>
  </div>
!

CSS 

              
                .box-wrapper {
  position: relative;
  max-width: 600px;
  height: 100vh;
  margin: 0 auto;
}

.box {
  width: 100%;
  text-align: center;
}

.text {
  padding: 20px;
}

.item {
  position: absolute;
  left: 50%;
  transform: translate(-50%, 0);
  width: 15px;
  height: 20px;
  margin: 0 auto;
  border-radius: 0% 70%;
  background: #914928;
  opacity: 0;
}

.item--horizontal {
  width: 20px;
  height: 15px;
}

.item--color2 {
  background: #ada137;
}

.item--color3 {
  background: #44681e;
}

@keyframes fall {
  from {
    opacity: 0;
    top: -50px;
  }

  20% {
    opacity: 1;
  }

  to {
    opacity: 1;
    top: 100vh;
  }
}

@keyframes sway1 {
  from {
    transform: translate3d(0px, 0px, 0px) rotate(0deg);
  }

  25% {
    transform: translate3d(-50px, 0px, 0px) rotate(-10deg);
  }

  75% {
    transform: translate3d(50px, 0px, 0px) rotate(10deg);
  }

  to {
    transform: translate3d(0px, 0px, 0px) rotate(0deg);
  }
}

@keyframes sway2 {
  from {
    transform: translate3d(0px, 0px, 0px) rotate(0deg);
  }

  25% {
    transform: translate3d(50px, 0px, 0px) rotate(10deg);
  }

  75% {
    transform: translate3d(-50px, 0px, 0px) rotate(-10deg);
  }

  to {
    transform: translate3d(0px, 0px, 0px) rotate(0deg);
  }
}

@keyframes sway3 {
  from {
    transform: translate3d(0px, 0px, 0px) rotate(40deg);
  }

  25% {
    transform: translate3d(50px, 0px, 0px) rotate(20deg);
  }

  75% {
    transform: translate3d(-50px, 0px, 0px) rotate(-20deg);
  }

  to {
    transform: translate3d(0px, 0px, 0px) rotate(0deg);
  }
}


.js-scroll-fall.is-animated .item:nth-child(1) {
  animation: fall 4s linear forwards, sway1 4s ease-in-out alternate;
}

.js-scroll-fall.is-animated .item:nth-child(2) {
  left: 56%;
  animation: fall 4s linear forwards, sway2 4s ease-in-out alternate;
  animation-delay: 0.3s;
}

.js-scroll-fall.is-animated .item:nth-child(3) {
  left: 40%;
  transform: translate3d(0px, 0px, 0px) rotate(90deg);
  animation: fall 4s linear forwards, sway1 4s ease-in-out alternate;
  animation-delay: 0.5s;
}

.js-scroll-fall.is-animated .item:nth-child(4) {
  left: 43.5%;
  transform: translate3d(0px, 0px, 0px) rotate(120deg);
  animation: fall 4s linear forwards, sway2 4s ease-in-out alternate;
  animation-delay: 0.7s;
}

.js-scroll-fall.is-animated .item:nth-child(5) {
  left: 54%;
  transform: translate3d(0px, 0px, 0px) rotate(60deg);
  animation: fall 4s linear forwards, sway1 4s ease-in-out alternate;
  animation-delay: 0.9s;
}

.js-scroll-fall.is-animated .item:nth-child(6) {
  left: 45%;
  transform: translate3d(0px, 0px, 0px) rotate(30deg);
  animation: fall 4s linear forwards, sway2 4s ease-in-out alternate;
  animation-delay: 1s;
}

.js-scroll-fall.is-animated .item:nth-child(7) {
  left: 50%;
  transform: translate3d(0px, 0px, 0px) rotate(-65deg);
  animation: fall 4s linear forwards, sway2 4s ease-in-out alternate;
  /* animation-delay: 1s; */
}

.js-scroll-fall.is-animated .item:nth-child(8) {
  left: 55%;
  transform: translate3d(0px, 0px, 0px) rotate(40deg);
  animation: fall 4s linear forwards, sway2 4s ease-in-out alternate;
  animation-delay: 0.2s;
}

.js-scroll-fall.is-animated .item:nth-child(9) {
  left: 70%;
  transform: translate3d(0px, 0px, 0px) rotate(-30deg);
  animation: fall 4s linear forwards, sway2 4s ease-in-out alternate;
  animation-delay: 0.5s;
}

.js-scroll-fall.is-animated .item:nth-child(10) {
  left: 65%;
  transform: translate3d(0px, 0px, 0px) rotate(-135deg);
  animation: fall 4s linear forwards, sway2 4s ease-in-out alternate;
  animation-delay: 0.7s;
}

.js-scroll-fall.is-animated .item:nth-child(11) {
  left: 60%;
  transform: translate3d(0px, 0px, 0px) rotate(135deg);
  animation: fall 4s linear forwards, sway3 4s ease-in-out alternate;
  animation-delay: 0.9s;
}

.js-scroll-fall.is-animated .item:nth-child(12) {
  left: 53%;
  transform: translate3d(0px, 0px, 0px) rotate(-120deg);
  animation: fall 4s linear forwards, sway1 4s ease-in-out alternate;
  animation-delay: 0.8s;
}

.js-scroll-fall.is-animated .item:nth-child(13) {
  left: 40%;
  transform: translate3d(0px, 0px, 0px) rotate(-120deg);
  animation: fall 4s linear forwards, sway3 4s ease-in-out alternate;
  animation-delay: 0.3s;
}

.js-scroll-fall.is-animated .item:nth-child(14) {
  left: 35%;
  transform: translate3d(0px, 0px, 0px) rotate(-120deg);
  animation: fall 4s linear forwards, sway1 4s ease-in-out alternate;
  animation-delay: 0.4s;
}

.js-scroll-fall.is-animated .item:nth-child(15) {
  left: 30%;
  transform: translate3d(0px, 0px, 0px) rotate(120deg);
  animation: fall 4s linear forwards, sway3 4s ease-in-out alternate;
  animation-delay: 0.3s;
}
!

JS 

              
                let targets = document.querySelectorAll('.js-scroll-fall'); //ターゲット要素
//スクロールイベント
window.addEventListener('scroll', function () {
  var scroll = window.scrollY; //スクロール量を取得
  var windowHeight = window.innerHeight; //画面の高さを取得
  for (let target of targets) { //ターゲット要素がある分、アニメーション用のクラスをつける処理を繰り返す
    var targetPos = target.getBoundingClientRect().top + scroll; //ターゲット要素の位置を取得
    if (scroll > targetPos - windowHeight) { //スクロール量 > ターゲット要素の位置
      target.classList.add('is-animated'); //is-animatedクラスを加える
    }
  }
});
!
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