CSS aims

I've been thinking a lot lately because of some related threads on other
lists:  Is it plausible to imagine a clean/reasonably complete decoupling
of presentation from structure without adding dubious amounts of
complexity?  I'm asking it here because i think it plays in some ways to
think about the direction of the platform present and future to look at
where to aim efforts at explanation and extensibility.

I guess my question is:  intentionally pretty vague because I'm interested
in hearing where you all think various answers and pieces being pondered
fit together in this general aim.