css/css-contain/content-visibility/content-visibility-animation-becomes-visible.html

<!DOCTYPE html>
<html class="reftest-wait">
<title>Test that making a content-visibility: hidden subtree visible restarts animations in it</title>
<link rel="author" title="Rob Buis" href="mailto:rbuis@igalia.com">
<link rel="help" href="https://drafts.csswg.org/css-contain-2/">
<link rel="match" href="content-visibility-animation-becomes-visible-ref.html">
<meta name="assert" content="animations in content-visibility: hidden subtree should restart once visible">

<script src="/common/reftest-wait.js"></script>

<style>
#container {
  content-visibility: hidden;
}
@keyframes fade {
  from { opacity: 0; transform: none; }
  to { opacity: 1; transform: translate(100px); }
}
#target {
  background: green;
  height: 100px;
  width: 100px;
}
.animate {
  animation: fade 1s linear 1 alternate;
  animation-fill-mode: forwards;
}
</style>
<body>
  <div id="container"></div>
</body>

<script>
function createAnimatingElement(name) {
  const container = document.getElementById('container');
  const target = document.createElement('div');
  container.appendChild(target);
  target.id = 'target';
  target.className = name;
  return target;
}

function runTest() {
  const container = document.getElementById('container');
  const target = createAnimatingElement('animate');
  container.style.contentVisibility = "visible";
  const listener = (e) => {
    takeScreenshot();
  };

  target.addEventListener("animationend", listener);
}
window.onload = () => requestAnimationFrame(() => requestAnimationFrame(runTest));
</script>