css/css-cascade/revert-layer-008.html

<!DOCTYPE html>
<title>CSS Cascade Layers: 'revert-layer' triggers a smooth transition</title>
<link rel="help" href="https://drafts.csswg.org/css-cascade-5/#revert-layer">
<link rel="author" href="mailto:xiaochengh@chromium.org">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<style>
@layer revert-to, revert-from;

@layer revert-from {
  #target {
    font-size: 10px;
    transition: font-size 2s linear -1s;
  }

  #target.reverted {
    font-size: revert-layer;
  }
}

@layer revert-to {
  #target { font-size: 20px; }
}
</style>

<div id="target"></div>

<script>
function raf() {
  return new Promise(resolve => requestAnimationFrame(resolve));
}
promise_test(async () => {
  const target = document.getElementById('target');
  getComputedStyle(target).getPropertyValue('font-size');

  await raf();
  target.classList.toggle('reverted');

  const result = getComputedStyle(target).getPropertyValue('font-size');
  assert_equals(result, '15px');
}, "'revert-layer' should revert font-size to 20px and trigger a smooth transition");
</script>