element');
  let errorEvent = await nextErrorPromise;
  assert_true(elem instanceof w2.HTMLUnknownElement, 'element should be an HTMLUnknownElement');
  assert_equals(errorEvent.target, w, `error event should target definition's global but instead targets ${event.currentTarget === w2 ? 'document\'s global' : 'test harness global'}`);
}, 'autonomous: exception thrown by spec due to invalid element state is reported to definition\'s global');

promise_test(async t => {
  assert_implements(supportsCustomizedBu