ritingMode, label, iframeWidth, iframeHeight) {
  const dialogSizesToTest = [["", ""], [dialogWidth.toString()+'px', dialogHeight.toString()+'px']];
  for (const dialogSizes of dialogSizesToTest) {
    const isDefaultSize = dialogSizes[0] == "";
    // This test doesn't make sense if the dialog sizes are default
    if (isDefaultSize && label == "dialog and viewport match") {
      continue;
    }

    async_test(t => {
      const iframe = document.createElement("iframe");
      iframe.src = `centering-ifr