It's taken a long time and I had to get outside help, but now the CSS validator actually validates CSS! Go ahead and accidentally use "display: bloc" (purposely misspelled) and the tool will now call you out on it. I also detect when you are using deprecated properties and sometimes suggest alternatives.
The next step is to kick it around a bit and parse our CSS. We only have approximately 1.3 megs of CSS (78k lines) which should get checked. I'll be working on refining the command-line utility during this time and hook it into our source code management tools. The entire UI team will be eventually exposed to this tool and I'm looking forward to their feedback. I need to know what more they would expect from this tool and how it could make life easier in other places.
After the initial feedback and during lulls in feature development, there is still the matter of adding block-level validation. For instance, if you use "opacity", you probably also want to use the IE-specific "filter" property.
I'll hang my head a bit and admit that I've added tons of code without tests. This must not be allowed to continue, so those missing tests will be written after we get our CSS files all checked. Documentation must also be written before releasing version 1.0.0.
So, now the challenge is up to you - does this tool do what you need?