mise("below-viewport");

  let has_window_loaded = false;

  async_test(t => {
    // Change the base URL and scroll down to load the deferred iframe.
    window.addEventListener("load", t.step_func(() => {
      const base = document.querySelector('base');
      base.href = '/invalid-url-where-no-subresources-exist/';
      has_window_loaded = true;
      below_viewport_iframe.element().scrollIntoView();
    }));

    below_viewport_iframe.promise.then(
      t.step_func_done(() => {
        assert_true(ha