HTML5 does. In fact the main question atm is about browser allowing accelerated canvas (as it is the browsers who are implementing the HTML5 specification, HTML5 by itself is nothing but a bunch of written documents establishing a standard).
So atm I believe Firefox and IE9 only (info to be confirmed) have hardware accelerated canvas support. For the others browsers, it is a matter of time before they catch up.
At the moment, browser games aren't as eye-candy as Construct's DirectX .EXEs
But once again, as time will go by, we can bet that browsers will be more and more reliable.
As for FPS consistancy, so far using Firefox 5 with my ATI Radeon HD 4800 I have had consistant 60 fps in every projects I've made/tested (Your C2 tests
, a topic that slipped to page 2).
For a little overview of HTML5 I'll link you to scirra's blog
and their last article about HTML5.