via @rcgordon - Original Nook ignored all CSS after a media query. iBooks would sometimes fail to process media queries after moving to a new chapter/doc (may be remedied)
via PubIt formatting guide -
"When defining styles in the CSS, margins should be set as “pixels”, not “ems”, or “% of ems”. This will optimize the viewing area regardless of selected font or type size.
The page margins should be set at 30 pixels on the top and sides, with 20 pixels on the bottom. Embedded fonts are allowed: we discourage their inclusion due to licensing issues and the added filesize.
Note that if you use Adobe InDesign to export an ePub and choose to encrypt embedded fonts, the resulting ePub will produce numerous errors in epubcheck.