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chromium/third_party/blink/web_tests/media/track/regions-webvtt/vtt-region-dom-layout.html 

<!DOCTYPE html>
<title>Tests default DOM layout structure for a VTTRegion.</title>
<script src="../../media-controls.js"></script>
<script src="../../../resources/testharness.js"></script>
<script src="../../../resources/testharnessreport.js"></script>
<script>
async_test(function() {
    var video = document.createElement('video');
    video.src = '../../content/test.ogv';
    video.autoplay = true;
    var testTrack = document.createElement('track');
    testTrack.onload = this.step_func(function() {
        video.oncanplaythrough = this.step_func_done(function() {
            var region = textTrackRegionElement(video);
            var container = textTrackRegionContainerElement(video);

            assert_equals(region.children.length, 1);
            assert_equals(region.children[0], container);
            assert_equals(internals.shadowPseudoId(region), '-webkit-media-text-track-region');
            assert_equals(internals.shadowPseudoId(container), '-webkit-media-text-track-region-container');
        });
    });
    testTrack.src = '../captions-webvtt/captions-regions.vtt';
    testTrack.kind = 'captions';
    testTrack.default = true;
    video.appendChild(testTrack);
});
</script>