<!DOCTYPE html>
<title>Scroll timeline with Web Animation using a scroller with overflow hidden</title>
<style>
  #box {
    width: 100px;
    height: 100px;
    background-color: green;
    transform: translate(0, 100px);
    opacity: 0.5;
    will-change: transform; /* force compositing */
  }

  #covered {
    width: 100px;
    height: 100px;
    background-color: red;
  }

  #scroller {
    overflow: hidden;
    height: 100px;
    width: 100px;
    will-change: transform; /* force compositing */
  }

  #contents {
    height: 1000px;
    width: 100%;
  }
</style>

<div id="box"></div>
<div id="covered"></div>
<div id="scroller">
  <div id="contents"></div>
</div>

<script>
  window.addEventListener('load', function() {
    // Move the scroller to halfway.
    const scroller = document.getElementById("scroller");
    const maxScroll = scroller.scrollHeight - scroller.clientHeight;
    scroller.scrollTop = 0.5 * maxScroll;
  });
</script>
