ssert_equals(document.fullscreenElement, A, 'first A request');
  assert_true(isTopLayer(A), 'A top layer');
  await blessTopLayer(A);
  try {
    await B.requestFullscreen();
  } catch (error) {
    assert_unreached('The second call to requestFullscreen rejected - it should be possible to put siblings into fullscreen together');
  }
  assert_equals(document.fullscreenElement, B, 'B request');
  assert_true(isTopLayer(B), 'B top layer');
  assert_true(isTopLayer(A), 'A still top layer');
  await blessTopLay