The <source> and <track> elements in videos are incorrectly written as <source></source> instead of <source />, see e.g. https://html5.validator.nu/?doc=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FWikipedia%3AVideos&submit=Check.
As the code actually looks good, and it only happens on user generated pages, not on file description pages, I guess that HTML tidy is the culprit.