css/css-pseudo/backdrop-animate.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS Pseudo-Elements Test: ::backdrop & web animations</title>
<link rel="author" title="Tim Nguyen" href="https://github.com/nt1m">
<link rel="help" href="https://fullscreen.spec.whatwg.org/#::backdrop-pseudo-element">
<link rel="help" href="https://drafts.csswg.org/web-animations-1/">
<meta name="assert" content="This test checks that ::backdrop can be animated with Web Animations." />
<div id="log"></div>
<dialog id="target">Dialog contents</dialog>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script>
const target = document.getElementById("target");
target.showModal();

test(function() {
  const options = {
    pseudoElement: "::backdrop",
    duration: 2,
    delay: -1,
  };
  const cs = getComputedStyle(target, "::backdrop");
  const anim = target.animate([
    {backgroundColor: "rgb(0, 100, 200)"},
    {backgroundColor: "rgb(200, 0, 100)"},
  ], options);
  this.add_cleanup(() => anim.cancel());
  assert_equals(cs.backgroundColor, "rgb(100, 50, 150)", "background-color");
}, "'backgroundColor' animation");
</script>