My initial thought is that the YouTube clip won’t auto-play within the formatting because the auto-play is a style of oneboxing? (that is, the forum does a bit of background magic, but won’t be able to if the link’s not on a line all by itself/not enclosed in other formatting)
I remember Brenna asking if tables would work in Hide Details, and that does. 
But I think that’s regular formatting within formatting, so as long as you don’t mix markdown and html too much it can handle it. Edit: I remember now. 
It was the answers to the Poetry Round in the Pub Quiz.
I’ll have a little play.
(and did you click on my coffee? )