edStyle(elem).backgroundImage;
  document.head.appendChild(base);
  this.add_cleanup(() => {
    document.head.removeChild(base);
    document.body.removeChild(elem);
  });
  style.setProperty("background-color", "green");
  assert_equals(getComputedStyle(elem).backgroundImage, originalComputedValue,
                "getComputedStyle(elem).backgroundImage after setting background-color");
  style.setProperty("background-image", "url(./)");
  assert_not_equals(getComputedStyle(elem).backgroundImage, original