me.
  let iframe = document.createElement("iframe");
  iframe.setAttribute("sandbox", "allow-scripts allow-same-origin");
  iframe.src = "/fetch/api/resources/infinite-slow-response.py";
  document.body.appendChild(iframe);

  // Remove sandbox flags. This should apply to documents committed from
  // navigations started after this instruction.
  iframe.removeAttribute("sandbox");
  iframe.src = "./resources/check-sandbox-flags.html";

  // The window is reused, but the new sandbox flags should be used.
  a