- Don't put in line-height unless you need it.
- Don't put in font-size in the body text unless you are doing small caps or something else that needs a different size. Small for offset text is a no-no.
- Don't use margins for the body of the eBook. Let the margins be set by the person reading using the program or Reader.
- Don't set a chapter header to have a 14% (or some other really stupid number) top margin.
- Don't use embedded fonts that look awful on an eInk screen.
- Don't use graphics to simulate text when you could don't have to.
- Don't use graphics that are of too low a resolution.
I think that's a good set of DONT'S to start with.