lement("z-index: 50; invalid");
  let style = elem.style;
  assert_equals(style.cssText, "z-index: 50;");
  // Create an observer for the element.
  let observer = new MutationObserver(function() {});
  observer.observe(elem, {attributes: true});
  function assert_no_record(action) {
    let records = observer.takeRecords();
    assert_equals(records.length, 0, "expect no record after " + action);
  }
  style.setProperty("z-index", "invalid");
  assert_no_record("setting invalid value to property");
  // Lo