yId('container');
  container.innerHTML = '<form><input type=submit><my-control>';
  const form = container.querySelector('form');
  const control = container.querySelector('my-control');
  control.tabIndex = 0;
  let invalidCount = 0;
  control.addEventListener('invalid', e => { ++invalidCount; });

  assert_true(control.i.checkValidity());
  assert_true(form.checkValidity());
  control.i.setValidity({valueMissing: true}, 'Please fill out this field');
  assert_false(form.checkValidity());
  assert_equals(