scroll-animations/css/scroll-timeline-default-iframe.html

<!DOCTYPE HTML>
<html class="reftest-wait">
<title>The default scroll() timeline in the iframe</title>
<link rel="help" href="https://drafts.csswg.org/scroll-animations-1/#scroll-notation">
<link rel="help" href="https://drafts.csswg.org/css-animations-2/#animation-timeline">
<meta name="assert" content="CSS animation correctly updates values when using the default scroll() timeline">
<link rel="match" href="scroll-timeline-default-iframe-ref.html">

<iframe id="target" width="400" height="400" srcdoc='
  <html>
  <style>
    @keyframes update {
      from { transform: translateY(0px); }
      to { transform: translateY(200px); }
    }
    html {
      min-height: 100%;
      padding-bottom: 100px;
    }
    #box {
      width: 100px;
      height: 100px;
      background-color: green;
      animation: update 1s linear;
      animation-timeline: scroll();
    }
    #covered {
      width: 100px;
      height: 100px;
      background-color: red;
    }

    * {
      margin-top: 0px;
      margin-bottom: 0px;
    }
  </style>
  <script src="/web-animations/testcommon.js"></script>
  <script>
    window.addEventListener("load", async function() {
      const scroller = document.scrollingElement;

      // Move the scroller to the halfway point.
      const maxScroll = scroller.scrollHeight - scroller.clientHeight;
      scroller.scrollTop = 0.5 * maxScroll;

      await waitForCompositorReady();
      await waitForNextFrame();
      await waitForNextFrame();

      window.parent.postMessage("success", "*");
    });
  </script>
  <body>
    <div id="box"></div>
    <div id="covered"></div>
  </body>
  </html>
'></iframe>
<script src="/web-animations/testcommon.js"></script>
<script>
  async function finishTest() {
    await waitForCompositorReady();
    await waitForNextFrame();
    await waitForNextFrame();
    document.documentElement.classList.remove("reftest-wait");
  }
  window.addEventListener("message", event => {
    if (event.data == "success") {
      finishTest();
    }
  }, false);
</script>