One of the greatest advantages of digital devices is their display flexibility... stylesheets shouldn't narrow the options, they should provide for more options. This allows users to select their reader based on its particular reading characteristics, not on whether or not a book is designed for it.
Forcing specific page and paragraph styles--proprietary formatting--would only result in books that would be unreadable on devices not optimized for that particular style... like taking an ebook specifically formatted for an iPad, and trying to read it on a cellphone screen. (And unfortunately, we're already seeing that in the newest iPad app magazine and textbook content, formatted for larger screens only, and unreadable on most smaller devices.)
CSS styles should all be universally set by a standards body and used identically by publishers. Individual reading devices should be able to decide how they will render the material, based on the CSS styles as modified for optimal display by that device, and tweaked by the individual user. All devices should be able to demonstrate that they are standards-compliant, so the consumer knows they will be able to read an ebook from any source.