Is there anything guaranteed by the ePub standard? For example, does the default have to be Serif and does there need to be a Sans Serif font?
Fonts are one of the more significant capabilities of the ePub standard, but ADE's "no user-level customization" approach makes even finding out basic information very difficult. With MOBI, for example, if you want to know which fonts are available on a device just bring up the list of user selectable fonts. MOBI isn't in ePub's league for fonts, but it is more transparent.
Do we need an ePub that tries to use all commonly available fonts? Unfortunately, the best approach would be to have two versions one with included fonts and the other without. However, relatively few fonts can legally be included in ePubs without a licence.