From:
http://www.idpf.org/epub/30/spec/epub30-changes.html
I note it says: "EPUB 3 defines a profile of CSS based on CSS 2.1 with added modules from CSS3, whereas EPUB 2 was based on a specific subset of CSS 2."
So I'm not sure where this idea that EPUB 2 is based on CSS 2.1 comes from, when it says there that it is based on a portion of CSS 2.