[kmill]

If you're willing to forgo the built-in drawing primitives, you can get access to the underlying buffer and draw directly to that. That way you avoid the CSS parser.

Edit: in particular, getImageData and putImageData.

[TazeTSchnitzel]

Yep, but then you forego hardware acceleration, so this is only reasonable at low resolutions.

If you want to do anything serious, WebGL's your only option.