t>


<script>
class MyCustomVideo extends HTMLVideoElement {
  constructor() {
  super();
  throw new Error();
  }
}
customElements.define('custom-video', MyCustomVideo, { extends: 'video' });
</script>
<video id=targetvideo is='custom-video'> <source></source> </video>
<script>
test(t => {
  let target = document.getElementById('targetvideo');
  assert_true(!!target);
  assert_equals(target.localName, 'video');
  assert_true(target instanceof MyCustomVideo);
  assert_true(target instanceof HTMLVideoElement