When using the <track> and <source> elements inside an <audio> or <video> element, closing </track> and </source> tags are automatically added.
However, if you run the resultant code through the W3 Validator, it gives a "Stray end tags" error about </track> and </source>.
So the desired output would be:
Updating the Sanitize gem appears to fix this, so let's do that! We'll also want to do a quick regression test of the HTML. (Instructions for that are on the wiki page titled HTML Tag Regression.)