I often just strip out all the CSS the publisher used, assuming the publisher used H1, H2, Hx tags for headings, P tags for paragraphs. That is, if the publisher used the typical HTML tags one SHOULD use. Then add in my own CSS, but I keep it as simple as possible. (The KISS concept.). Unfortunately a few publishers often use tons of DIV and SPAN tags, and use P tags for headings, paragraphs, and pretty much everything. They use class parameters to change everything, just about. Those publishers are total idiots, and modifying their crap code takes forever!

If you use the left-margin parameter for the P tag, but later in the CSS there is another left-margin parameter for the P tag, then yours can be overwritten. That is why I try to clean house and get rid of all the publisher poop that shouldn't be in HTML designed for eBooks.