ndow.location.href, (new URL(startURL, location.href)).href, "Iframe must be navigated away from the initial about:blank document");
  assert_equals(iframe.contentWindow.onloadFired, undefined, "onload must not yet have fired");

  iframe.src = afterReplacementURL;

  await checkSentinelIframe(t, sentinelIframe);
  assert_equals(history.length, startingHistoryLength, "history.length must not change after checking the sentinel iframe");
}, "Replace before load, triggered by setting iframeElement.src");
</scr