Speaking of CSS, that’s where you want to do all your styling. It’s the preferred way to stylize things in both XHTML and the ePub standard. (There are a very few reader programs out there that won’t recognize CSS. Mot of them do by now though, and if you keep the HTML to the basics, the book will still be readable if the reader program doesn’t recognize CSS.
via .epub eBooks Tutorial – Part 1: Format Your Source Content.