Couldn't you do the post-processing using CSS filters?
Note these are still pretty new. Seems you need Chromium 18 for the above samples to work. But they would appear to be on a path for inclusion in HTML5.
Forgive me if this has been brought up before, I'm fairly new to the mailing list.
I've been experimenting with the idea of creating a browser extension that would allow users to inject limited post-process effects into any WebGL canvas. Essentially the extension would work by redirecting gl.bindFramebuffer to force scenes to render into a texture, then just before end-of-frame render that texture with an arbitrary effect shader. The target effect would be FXAA but any image-based effect would work (greyscale, edge detection, etc.)