HTML5 is no longer just a buzzword, people actually get things done using modern web technologies that are colloquially and not entirely accurately called HTML5. Well, OK, HTML5 is still a buzzword, but the point is, it's not just buzz.
One of the ways used by the big players interested in getting more and more people to use HTML5 & co. and the web in general for apps, that would be Google and Facebook, is to talk about gaming.
There's a good reason for this, games are inherently fun and cool and they always get more attention than some office collaboration app or something to the effect.
But it's not just that, games are always at the forefront of what technology and hardware can do. Put simply, if you can build a game with HTML5 and associated technologies, you can be sure you can build pretty much any other type of app.
The web does have quite a lot to offer to game developers at this point, starting with HTML5 Canvas for 2D graphics, WebGL for full, GPU accelerated 3D graphics, Web Audio API for real-time audio processing, Websockets for asynchronous networking.
Google, Facebook, Mozilla and others have been touting these technologies for a while. A few months ago, a team at Google set out to put its money where its mouth is, i.e. actually build a game that uses the web rather than just encourage people to do it.
This is how GRITS was created, a player vs. player, top-down robot shooting game that was showcased during a session on HTML5 gaming at Google I/O last week.
It's a simple game: you get a mech robot you can customize to a degree, there are all sorts of power-ups and you have to shoot and destroy other players' robots or be shot by them.
You can go play the game right now, if the servers aren't too busy, or check out the full source code, including most assets, over at Google Code. Of course, you can watch the full session to see how it was built and what the challenges were.