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

<!DOCTYPE html>
<title>TextTrack "id" attribute</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<video>
    <track id="LoremIpsum" src="resources/captions-fast.vtt" default>
    <script>
    test(function() {
        var video = document.querySelector("video");
        var track = document.querySelector("track");
        var textTrack = track.track;

        // Test default attribute value.
        assert_equals(textTrack.id, "LoremIpsum");
        assert_equals(video.textTracks[0].id, "LoremIpsum");

        // Make sure we can look up tracks by id.
        assert_equals(video.textTracks.getTrackById("LoremIpsum"), textTrack);

        // Test that it's readonly.
        textTrack.id = "newvalue";
        assert_equals(textTrack.id, "LoremIpsum");
    });
    </script>
</video>