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chromium/third_party/blink/web_tests/media/track/track-cue-rendering-vertical.html 

<!DOCTYPE html>
<title>Test, rendering vertical line-positioned cues with Japanese text.</title>
<video>
    <track src="captions-webvtt/captions-snap-to-lines-set.vtt" kind="captions" default>
</video>
<script>
var video = document.querySelector("video");
video.src = "../content/test.ogv";

testRunner.waitUntilDone();
video.onseeked = function() {
    // Wait for one rAF to ensure the thumbnail is onscreen, which helps reduce
    // flaky failures.
    if (window.testRunner)
        requestAnimationFrame(_ => testRunner.notifyDone());
}

video.currentTime = 0.75;
</script>