Boilerplate CSS used in EPUBs I make. Created for Japanese light novels, though I imagine these could apply elsewhere.
epub3.css: Catered toward EPUB3 publications. Uses some CSS3 properties, and styles some things based on EPUB 3 Structural Semantics. Includes fallback measures for reading systems that only support ePub2.
epub2.css: Use in ePub2 or EPUB3. Stripped-down version of epub3.css. Uses simpler selectors and properties that are more likely to be supported. Most applications default to creating ePub2 files, so use this if you are unsure.