FM's
How to create a signature says "If your default message format is set to allow rich text formatting, the signature text box will also be rich text,...". Would it be reasonable then to interpret that if you had selected the advanced compose preference's "plain text" or "When replying, use the same format as the original message"(and the original message is plain text) that it would just ignore the sending identity's HTML sig formatting when replying? I didn't test this so it's just a guess.