chromium/third_party/blink/web_tests/external/wpt/css/css-animations/flip-running-animation-via-variable.html

<!DOCTYPE html>
<html class="reftest-wait">
<title>Transform animation under large scale</title>
<link rel="author" title="Kevin Ellis" href="mailto:[email protected]">
<link rel="help" href="https://crbug.com/1221622">
<link rel="match" href="flip-running-animation-via-variable-ref.html">
<script src="/common/reftest-wait.js"></script>
<style>
@keyframes spin {
  0%   { transform: scaleX(var(--scale)) rotate(0deg); }
  100% { transform: scaleX(var(--scale)) rotate(180deg); }
}

html {
  --scale:  1;
}

html.tweaked {
  --scale:  -1;
}

#container {
  display: inline-block;
  margin-left:  150px;
  padding:  0;
  /* Force animation to be effectively frozen at 50% progress. */
  animation: spin 1000000s cubic-bezier(0, 1, 1, 0)  -500000s;
}

#block-1 {
  background: blue;
  height: 200px;
  width:  100px;
}

#block-2 {
  background: green;
  height: 100px;
  width: 100px;
}
</style>

<body>
  <div id="container">
    <div id="block-1"></div>
    <div id="block-2"></div>
  </div>
</body>

<script>
window.onload = () => {
  const anim = document.getAnimations()[0];
  anim.ready.then(() => {
    requestAnimationFrame(() => {
      requestAnimationFrame(() => {
        document.querySelector('html').classList.add('tweaked');
        requestAnimationFrame(() => {
          requestAnimationFrame(() => {
            takeScreenshot();
          });
        });
      });
    });
  });
};
</script>
</html>