c((entryList, observer) => {
      assert_equals(entryList.getEntries().length, 1);
      entryList.getEntries().forEach(entry => {
        assert_equals(entry.entryType, "element");
        const pathname = window.location.origin + '/element-timing/resources/square20.jpg';
        checkElement(entry, pathname, 'my_image', 'my_id', beforeRender, img);
        checkNaturalSize(entry, 20, 20);
        observer.disconnect();
        t.done();
      });
    })).observe({
      type: "element",
      buffered: t