Apparently not too much. This is the latest comparison chart:
http://www.findmebyip.com/litmus/
It really doesnt matter which browser you are using, Firefox, Safari, Chrome, IE, or Opera – NONE of them are compatible at this point. Sure some of them are somewhat compatible but none are even close to fully supporting most options. What does this mean for HTML5? It means that at this point, I would be very weary of developing for it. Wait until its fully compatible then do it.
Essentially if all new browsers from today on were 100% compatible I would still wait some time before developing for HTML5. Why? Most users to not upgrade their browsers unless they are set up update automatically. Thus you would be developing something that more than likely only your tech savvy users are able to view.