Is there a description somewhere of the text formatting codes?
Incidentally, I tried applying underline to some text in one of my posts, and the button put markups of letter 'u' in square brackets. When I posted it, those markups were actually shown instead of putting underlines on.
(Firefox on Ubuntu).
I'll try the underlining again here (it looks OK in the preview).
Update: It worked this time. Puzzled!