f HTMLElement, 'A resolved custom element must be an instance of HTMLElement');
    assert_false(customElement instanceof HTMLUnknownElement, 'A resolved custom element must NOT be an instance of HTMLUnknownElement');
    assert_true(customElement instanceof MyCustomElement, 'A resolved custom element must be an instance of that custom element');
    assert_equals(customElement.localName, 'my-custom-element');
    assert_equals(customElement.namespaceURI, 'http://www.w3.org/1999/xhtml', 'A custom element HT