ment(w, class extends w.HTMLElement {
        constructor() { super(); this.appendChild(w.document.createElement('a')); }
      });
    }, 'should throw if it has a child element');
    assert_throws(expectNotSupportedError, () => {
      define_and_create_element(w, class extends w.HTMLElement {
        constructor() { super(); this.appendChild(w.document.createTextNode('a')); }
      });
    }, 'should throw if it has a child text node');
  }, `${description}6.1.5. If result has children, then throw a Not