IIRC, FBReader180 supports <b>, <i>, <strong> and <em>.
Basic support for styling with CSS styles was only added to the latest version from SciFiFan (FBReader180h). It supports font-weight:bold and font-style:italic. I don't know if these changes were integrated into the better known version of FBReader180 from GrayNM.
Edit:
SciFiFans version:
http://pbook.s3.amazonaws.com/fbreader_180h.zip (Direct download, I'm not aware of a home page.)
GrayNMs version:
http://code.google.com/p/fbreader-pockebook/