Well for starters even getting a canvas based UI working that supports all DPIs, all aspect ratios, live resize, clicks, and multi-touches is non-trivial.
Not to mention text input.
And unless we do some strange CSS tricks to force acceleration, there isn't even the guarantee that canvas is doing hardware acceleratd drawings.
Audio is what was breaking. Also WebGL outperforms canvas, and it’s more complex than drawing a bitmap to the screen.