Thank you for making browsers even less uniform than before...
The WHATWG spec precisely describes how to parse HTML. No previous HTML spec has done this. So browsers now have a target to converge upon. It won't happen overnight, but over time it will gradually make the web a more consistent and reliable platform to develop on.
One reason why previous W3C HTML specs were short is they left out most of the detail needed to achieve interoperability. The WHATWG have done something awesome: they reverse engineered how browsers currently parse HTML and documented it. They also spec'd undocumented but essential parts of the platform like document.write, innerHTML, Window Object, XmlHTTPRequest, <canvas> etc.
This "HTML" parsing algorithm is used in the new HTML validator. Firefox 4 implements this exact parsing algorithm. Opera, Safari and Chrome are converging on it. And Microsoft are involved - and submitting test cases!
Browsers are adding new features at a faster rate than any time in web history. This is a great time!