That the canvas API is implemented with OpenGL is very impressive. CocoonJS [1] (a similar project for Android, as noted in the comments below) seems to have taken the same approach.
On a completely tangential note, given that the main desktop browsers now sport hardware accelerated canvas implementations, is there a performance difference in 2D graphics rendered with canvas vs. WebGL?
Canvas is directly implemented in gl (or directx) in most browsers at this point. WebGL is still faster like the poster above says because you as the programmer an can make optimizations that are not possible with the canvas api like drawing many things with one call.
On a completely tangential note, given that the main desktop browsers now sport hardware accelerated canvas implementations, is there a performance difference in 2D graphics rendered with canvas vs. WebGL?
[1] http://ludei.com/tech/cocoonjs