Which standard?
Which standard are we talking about here? HTML 5 is not a standard yet and, as it stands, whilst the video element is part of the markup language the video encoding itself hasn't been specified.
We're currently at the stage where every browser maker can implement the video tag and claim "HTML5 support" whilst one browsers uses <video> to stream Ogg, another H.264 and another VP8 - and ALL of them can rightly claim to "support HTML5" in it's current form.
Eventually there will be consensus on which video codec to use (this announcement has probably given VP8 a massive shot in the arm) and that will become part of the HTML 5 "standard" - that's how web standards evolve, they're not just magically spawned good-to-go, they start as an idea, move to "working draft", "last call" and finally "recommendation".
Big chunks of HTML 5 are still working drafts - claiming "HTML 5 " support at the moment is a bit like sticking a big BETA label onto your browser... sounds perfect for Google then ;)