Cssutils are based upon and partly implement the following
specifications (DOM only, not any rendering facilities):

CSS 2.1 Rules and properties are defined here

CSS 2.1 Errata A few erratas, mainly the definition of CHARSET_SYM
tokens

MediaQueries MediaQueries are part of stylesheets.MediaList since
v0.9.4, used in @import and @media rules.

Namespaces Added in v0.9.1 and updated to definition in CSSOM in v0.9.4

Selectors The selector syntax defined here (and not in CSS 2.1) should
be parsable with cssutils (should mind though ;) )

DOM Level 2 Style CSS DOM for package css

DOM Level 2 Style Stylesheets DOM for package stylesheets

CSSOM A few details (mainly the NamespaceRule DOM) is taken from here.
Plan is to move implementation to the stuff defined here which is newer
but still no REC so might change in the future