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chromium/third_party/blink/web_tests/external/wpt/html/semantics/embedded-content/media-elements/track/track-element/track-mode-triggers-loading.html 

<!DOCTYPE html>
<title>A "metadata" track does not load automatically, but it does load when the mode is changed</title>
<script src="/common/media.js"></script>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<video>
    <track kind="metadata" src="resources/metadata.vtt">
</video>
<script>
async_test(function(t) {
    var video = document.querySelector("video");

    // Check initial metadata track state.
    var track = document.querySelectorAll("track")[0];
    assert_equals(track.readyState, HTMLTrackElement.NONE);
    assert_equals(video.textTracks[0].mode, "disabled");

    video.src = getVideoURI("/media/test");
    video.oncanplaythrough = t.step_func(canplaythrough);
    track.onload = t.step_func_done(trackLoaded);

    function trackLoaded() {
        assert_equals(track.readyState, HTMLTrackElement.LOADED);
        assert_equals(track.track.mode, "hidden");
        assert_equals(video.textTracks[0].cues.length, 12);
        assert_equals(video.textTracks[0].cues[11].startTime, 22);
    }

    function canplaythrough() {
        assert_equals(track.readyState, HTMLTrackElement.NONE);
        assert_equals(video.textTracks[0].mode, "disabled");
        assert_equals(video.textTracks[0].cues, null);
        // Change metadata track mode so it loads.
        video.textTracks[0].mode = "hidden";
    }
});
</script>