We've long established that W3C compliance of websites is not a ranking factor per se, but what about browser compatibility?
Say you have a website that's optimized for modern browsers, empoying techniques such as CSS sprites, and you have no fallback methods for older browsers in place (for whatever reason). In the case of CSS sprites, for example, your images won't load for visitors using IE 7 or older.
In lieu of Google's never-ending quest to give their users the best possible experience, and supposing they're capable of detecting the above, wouldn't it make sense for them to lower your rankings in searches performed by users of such older browsers?
I'm just throwing this out there even though I just realized this is indirectly embedded in Panda. After all, if you don't support older browsers, any consequentially negative user experience is likely to have an effect on your engagement metrics such as bounce rate and time on site.