One nitpick against that comes readily to mind: we've already had people griping fairly recently that they can't just use markdown (or the like) to format posts and that the (relatively limited) PM HTML dialect required now is "just too hard" for noobs. I'm not thinking that they're going to be willing (to say nothing of able . . . *cough*) to correctly use even more specific use case HTML5-new-shinyness.
That aside though I'd probably agree in principle that were changes to the markup to be added this would be a reasonable bit of HTML5 to allow.
The cake is a lie.
The cake is a lie.
The cake is a lie.
