tform');
  assert_true(!!target);
  assert_equals(target.localName, 'form');
  assert_true(target instanceof MyCustomForm);
  assert_true(target instanceof HTMLFormElement);
  assert_equals(target.children.length, 2);
}, 'Throwing exception in customized built-in constructor should not crash and should return correct element type (form)');
</script>

<script>
class MyInput extends HTMLInputElement { };
customElements.define('my-input', MyInput, { extends: 'input' });
</script>
<input id=customized-input is=