chromium/third_party/blink/web_tests/fullscreen/full-screen-iframe-allowed-video.html

<!DOCTYPE html>
<script>
    if (window.internals)
        runPixelTests = internals.runtimeFlags.forceOverlayFullscreenVideoEnabled;
</script>
<script src="full-screen-test.js"></script>
<script>
    var initialized = false;
    var fullscreened = false;
    window.onmessage = function(messageEvent) {
        if (!initialized) {
            // video has been loaded and messaged us.
            // Send click to center of iframe.
            iframe = document.getElementById("frame");
            x = iframe.offsetLeft + iframe.offsetWidth / 2;
            y = iframe.offsetTop + iframe.offsetHeight / 2;
            if (window.eventSender) {
                eventSender.mouseMoveTo(x, y);
                eventSender.mouseDown();
                eventSender.mouseUp();
            }
            initialized = true;
            return;
        }

        if (!fullscreened) {
            // The video has been fullscreened.
            consoleWrite(messageEvent.data);
            fullscreened = true;
            endTest();
            return;
        }
    }
</script>
Video inside an iframe is allowed to enter fullscreen</p>
<iframe id="frame" src="resources/video.html" width="320" height="240" allowfullscreen></iframe>