It's also worth noting that, as Jellby first pointed out to me several months ago (to my initial disbelief
), the MOBI/PRC format doesn't natively use or understand any CSS at all. Applications (such as Kindlegen, Mobipocket Creator and Calibre) that convert other formats to MOBI/PRC have to interpret the CSS and convert it into the closest approximation available in the rather basic HTML (plus a few proprietary tags) used in the MOBI/PRC format. This is the main reason that the formatting options available in a MOBI book are substantially less than what you can do in an EPUB book.