That doesn't really make sense. HTML 5 isn't a standard yet. And it isn't being written so that browsers will then implement it. Rather it is being written for the most part around what browsers have already implemented.
I think it is fair to say that Firefox 3 has the most complete implementation of the proposed HTML 5 working draft of any browser. Webkit is also doing pretty well. IE has implemented many important parts. But we are a long way from having a final specification, never mind having that implemented in every browser. And then that's a long way from actually being able to use any of it and abandon users on old browsers.