Funny you should mention that - much of the work already done was to document and work out implementations of Internet Explorer's behaviour. Wherever practicable, the current implementation is favoured as the recommended implementation (ie. not if the current behaviour is to crash the browser, for example!).
HTML 5 is not an impementation of IE de facto standards as such, but it is aimed to be compatible with current browser behaviour, rather than with the complete break with current browsers previously proposed by the XHTML 2.0 draft.
So HTML 5 will be much "saner" with browsers, seeing as its development it driven by the browser developers themselves (Opera, Apple, Mozilla and now Microsoft).