I really like:
- Using CSS-like syntax in the -mw namespace in general (especially because, like you say, it allows integrating it where it's intended not above the rule and have the processor figure it out, thus limiting the usage)
- Introduction of
-mw-file - Moving processing into 1 unit, instead of multiple (right now having embed in CSSMin is fine, but if we're extending it further it should be moved out).
I'm personally not a fan of the hierarchy section, although I haven't investigated that a lot yet.