media-element-move-to-new-document-assert.html

<!DOCTYPE html>
<p>Test for failing EventDispatchForbiddenScope assert when moving a
video with text track between documents.</p>
<video></video>
<iframe></iframe>
<script>
if (window.testRunner) {
  testRunner.dumpAsText();
  testRunner.waitUntilDone();
}

window.onload = function () {
    var video = document.querySelector("video");
    var track = document.createElement("track");

    // Note: The order of setting the text track mode and appending the
    // track element is important, in terms of implementation and spec:
    // https://www.w3.org/Bugs/Public/show_bug.cgi?id=28973
    track.track.mode = "showing";
    video.appendChild(track);

    // Move the video element to another document.
    var iframe = document.querySelector("iframe");
    iframe.contentDocument.body.appendChild(video);
    if (window.testRunner)
        testRunner.notifyDone();
};
</script>

chromium/third_party/blink/web_tests/media/track/media-element-move-to-new-document-assert.html