ine('custom-p', MyCustomParagraph, { extends: 'p' });
</script>
<p id=targetp is='custom-p'></p>
<script>
test(t => {
  let target = document.getElementById('targetp');
  assert_true(!!target);
  assert_equals(target.localName, 'p');
  assert_true(target instanceof MyCustomParagraph);
  assert_true(target instanceof HTMLParagraphElement);
  assert_equals(target.childNodes.length, 1);
  assert_equals(target.textContent, 'PASS');
}, 'Appending children in customized built-in constructor should work');
</scrip