discussion seems to have died down here but I'd like to bring up another issue
In WebGL land we have creation attributes on the drawingbuffer made for a canvas. Example gl = canvas.getContext("webgl", { preserveDrawingBuffer: false }); We're working out the details on how to set those options for the case where we have 1 context and multiple canvases. The particular option above would apparently be a huge perf win for canvas 2d for mobile. Which suggests that whatever API is decided on it would be nice if it worked for both APIs the same.