The
UTTER minimum is to not have one at all. It's not required.
BUT your books would look....ummmm....
horrendous. Unless you decided to totally go against best practice and put any/all styling inline (in your html)...
You can have a really good, really compact, minimalist css file that really makes your book look good. It just takes practice/experience. I would highly recommend
NOT ignoring the possibilities of css in your ePub reinvention journey.
edit: ooops...didn't see page 2 lol. What
they said...