I think generally speaking, reading the spec is useful when trying to figure out the details of more complex topics, I think this holds true for most languages. I used to pull out the C spec regularly, and even the c++ draft spec on occasion, it gets better for smaller languages, for Scheme for instance, I would pull out the spec rather quite often.

Since I imagine the CSS spec is relatively small (compared to say C++) I imagine it would be useful for solving more adavnced use cases.