lso trigger only one mutation record.
  style.setProperty("margin", "1px");
  assert_record_with_old_value("", "adding shorthand property to CSS declaration block");
  style.removeProperty("margin");
  assert_record_with_old_value("margin: 1px;", "removing shorthand property from CSS declaration block");
  // Final sanity check.
  assert_equals(elem.getAttribute("style"), "");
}, "Changes to CSS declaration block should queue mutation record for style attribute");

test(function() {
  let elem = createTestE