ame>
<script>
  let iframe = document.querySelector("iframe");

  async_test(t => {
    let originDoc = iframe.contentDocument;
    window.addEventListener("load", t.step_func_done(() => {
      assert_equals(iframe.contentDocument, originDoc, "contentDocument shouldn't be changed");
    }));
  }, "iframe.contentDocument should not be changed");

  async_test(t => {
    iframe.addEventListener("load", t.unreached_func());
    window.addEventListener("load", () => t.done());
  }, "load event of iframe should