Quake II Running in HTML5
Check this out.
That’s Quake II running in HTML5 with WebGL, Canvas, and JavaScript. No Silverlight, no Flash, 20-60fps depending on the horsepower of the machine. They are having some licensing issues with sharing the Quake maps, but you can run similar short demos here*.
I think it’s safe to say canvas can, as a general rule, be used for the kind of complex rendering, animation, and UI you would use Silverlight or Flash for. Add in WebGL for 3D rendering and the possibilities really start to take off.
*To get them to work in Ubuntu I had to start chromium with –enable-webgl and –no-sandbox.