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chromium/third_party/blink/web_tests/external/wpt/html/semantics/embedded-content/media-elements/track/track-element/track-cue-inline.html 

<!DOCTYPE html>
<title>Add a track and change its mode through JS</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<video>
    <source src="/media/test.mp4" type="video/mp4">
    <source src="/media/test.webm" type="video/webm">
</video>
<script>
test(function() {
    var video = document.querySelector('video');
    var track = video.addTextTrack('captions', 'English', 'en');
    track.addCue(new VTTCue(0.0, 10.0, 'wow wow'));
    track.mode = 'showing';
});
</script>