With the SRT file type, all (HTML) formatting should be avoided, for reasons of compatibility. It may work and it may not work - and then displaying those tags as plain text, very bad.
Also, I would think twice before using any text formatting anyway. Subtitles are meant to support the movie and should be as invisible and unnoticed as possible. The simpler, the better.
But for what you want, consider the SSA file type.