er the
 * task for media event.
 */
async_test(function(t) {
    let video = document.createElement("video");
    video.play();
    video.onplay = t.step_func(startedPlay);

    // When we create a text track element, it queue a task to run automatic
    // text track selection later.
    let track = document.createElement("track");
    track.default = true;
    video.appendChild(track);
    assert_equals(track.track.mode, "disabled", "Text track's mode is disabled by default.");

    function startedPlay()