@Jellby
My opinion is that setting up
font-family, lateral margins, line-height only once in the body let them be easily modifiable either by current ebook-readers like KoboGlo or by opening the CSS.
Sorry for the Nook, I was joking.
@bfollowell
Thanks for sharing.
About relative units
Yes
em is certainly a unit I will consider for margins, paddings and font-size.
I've found that % -which looks externally very handy- may yield inconsistent results (some readers or apps fail to take them precisely into account) for margins and paddings. It's perfect for font-size.
Other attributes
I personnaly do not setup
text-align and
font-size in the
body but it's your choice.
Could you comment further on why you use these following attributes. I may have missed something (and I certainly missed the DiapDealer comment above).

-
display:block;
-
page-break-before:always;