I think "just" going for CSS 2 would be a mistake. In my opinion, it would be preferable take new concepts that are simple to implement and simplify life for everybody (e.g. vh, vw units and flexbox, box-sizing). Goal should be to get a minimal deterministic rendering engine for content that's easily written manually with a minimal subset of modern XHLTML+CSS.

Re:XHMTL: as somebody pointed out here, there are rules to "normalize" unbalanced HTML5, but they have to be implemented and add to the mountain of "implicit" knowledge one has to have and implement...