Jeff has it correct: browsers don't give a rat's. Any browser who has shipped with an HTML5 parser (and they're all partial since HTML5 is still in draft) will see any page served as text/html as "HTML5" no matter what the doctype claims. Any who hasn't, will use their HTML4 parser (yes, even XHTML... browsers ignore those closing slashes and other silliness too).
Validators care though. But even that only matters if you don't like wading through "errors" that you understand aren't really errors, or if you're worried about missing a stylistic error you were hoping a stricter validator would call you on (like using "checked" instead of 'checked="checked"' for example).
As a card-carrying dirty hippie FLOSS whacko, I f*ckin HATE Flash and the way Adobe's hatred and bigotry against Linux has caused us unreasonable problems with videos on our "open sores" internets. Screw Adobe with the longest, most barbed pole you can find... maybe one of those things cops lay on the road to make cars' tires go flat. One company controlling how a form of media can be used and played? With closed software that [gets hacked and [url=http://www.wired.com/epicenter/2009/08/you-deleted-your-cookies-think-again/]uses its own cookies](http://www.feross.org/webcam-spy/) (separate from deleteable browser cookies) so they can spy on us more? That sucks, and that's, like, my opinion, man.
* all software has bugs and can get hacked. It's that I like more eyes and immediate response and fix like I get from the dirty hippies than waiting on Adobe to decide when they'll get around to my OS...
Canvas is getting retarded though, since the folks pushing it into the spec keep saying "all it does is make a picture by colouring pixels on the screen using coordinates, don't use this to make GUIs and things" and then what do people do? Make (naturally inaccessible at this point) GUIs and things with it. As everyone should have expected, being all new and pretty and coupled with the HTML5 hype and coming from the shiny Apple guys and, most importantly, not as scary-looking as SVG. (and for games, not as slow, I'll give them that)
(and for those of you about to pipe in with the "fallback content" Bull, that's NOSCRIPT's job!)