css/css-transitions/pseudo-elements-002.html

<!DOCTYPE html>
<title>CSS Transitions Test: Transition pseudo element with ancestor display change</title>
<link rel="author" title="Rune Lillesveen" href="mailto:futhark@chromium.org">
<link rel="help" href="https://drafts.csswg.org/css-transitions/#starting">
<link rel="help" href="https://drafts.csswg.org/css-transitions/#transition-property-property">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  #inner::before {
    content: "This text should transition from red to green.";
    height: 100px;
    transition: height steps(2, start) 1s;
  }
  .flex #inner::before {
    height: 300px;
  }
  .flex { display: flex }
</style>
<div id="outer">
  <div id="inner"></div>
</div>
<script>
  test(() => {
      assert_equals(getComputedStyle(inner, "::before").height, "100px");
      outer.className = "flex";
      assert_equals(getComputedStyle(inner, "::before").height, "200px");
  }, "Check that transitions run on a pseudo element whose ancestor changes display type.");
</script>