 assert_attr(value, action) {
    assert_equals(elem.getAttribute("style"), value, "style attribute after " + action);
  }
  style.setProperty("z-index", "31");
  assert_attr("z-index: 31;", "changing property in CSS declaration block");
  style.removeProperty("z-index");
  assert_attr("", "removing property from CSS declaration block");
  style.setProperty("position", "absolute");
  assert_attr("position: absolute;", "adding property to CSS declaration block");
  style.cssText = "z-index: 32;";
  assert_at