t_equals(getComputedStyle(redIframeSpan).color, "rgb(0, 0, 0)");
  assert_equals(getComputedStyle(blueIframeSpan).color, "rgb(0, 0, 0)");
  assert_equals(getComputedStyle(whiteIframeSpan).color, "rgb(0, 0, 0)");
  assert_equals(getComputedStyle(yellowIframeSpan).color, "rgb(0, 0, 0)");

  const iframePlainSheet = new iframe.contentWindow.CSSStyleSheet();
  const iframeRedStyleSheetPromise = iframePlainSheet.replace(redStyleTexts[0]);
  return iframeRedStyleSheetPromise.then(function(iframeRedStyleSheet) {
 