styleDefDecl identifies a "formal definition language"
A legal value is "free" and it means "Informal free text description". So the formal language is informal.
The note says "If no formal definition language is supplied or implied, the default assumption is that CSS is in use."
This leads to the odd implication that if @scheme = "free", that is, if the scheme is informal not formal, then CSS is in use.
Shouldn't "css" and "xslfo" be possible values, with others allowed (wordperfect?, troff? :) ). Even is a user-defined formal language is specified, presumably that language has a name name.