t emptyStyleSheet = new CSSStyleSheet();
  document.adoptedStyleSheets = [emptyStyleSheet];
  assert_equals(getComputedStyle(greenSpan).color, "rgb(0, 0, 0)");
  assert_equals(getComputedStyle(redSpan).color, "rgb(0, 0, 0)");
  assert_equals(getComputedStyle(blueSpan).color, "rgb(0, 0, 0)");
  assert_equals(getComputedStyle(whiteSpan).color, "rgb(0, 0, 0)");
  assert_equals(getComputedStyle(yellowSpan).color, "rgb(0, 0, 0)");

  assert_equals(getComputedStyle(greenIframeSpan).color, "rgb(0, 0, 0)");
  asser