css/selectors/x-pseudo-element.html

<!doctype html>
<title>CSS Test: 'x-' prefixed pseudo-elements don't parse correctly</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="author" title="Mozilla" href="https://www.mozilla.org">
<link rel="help" href="https://drafts.csswg.org/selectors-4/#pseudo-elements">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  p {
    color: green;
  }
  ::x-something-nobody-would-think-of, p {
    color: red;
  }
</style>
<p>Should be green
<script>
  test(function() {
    let p = document.querySelector('p');
    assert_equals(getComputedStyle(p).color, "rgb(0, 128, 0)");
  }, "x-prefixed pseudo-elements should make the whole rule invalid")
</script>