It's not in CSS 2.1, but it is in CSS 2 (
http://www.w3.org/TR/2008/REC-CSS2-2...t-descriptions). And, in fact, ePub 2.0.1 is based on CSS 2:
"
This specification defines a style language based on CSS 2. (Note that the CSS 2.1 specification is currently still at "Working Draft" status.)"
Maybe Sigil validation should default at CSS 2 instead.