YouTube has been releasing videos in the WebM format. I remember using one downloader that offered to output in number of available formats, perhaps with conversion, but perhaps they were available as such from the source. I don't know.
What I do know is that DVD Flick doesn't recognize those WebM files. That creates a problem, as I share TedEd videos on DVD with students.
If not another downloader, I'm going to need come kind of converter.