esn't load a new document, so
  // it will stay on the initial empty document.
  const iframe = insertIframeWith204Src(t);
  assert_equals(history.length, startingHistoryLength,
    "Inserting iframe with src set to URL that doesn't load a new document must not change history.length");

  // Do a history.pushState() to about:blank#foo.
  let pushURL = "about:blank#foo";
  iframe.contentWindow.history.pushState({}, "title", pushURL);
  assert_equals(iframe.contentWindow.location.href, pushURL);
  assert_equa