We already do this by exposing the canvas surface with a semaphore lock. The browser can flip the surface to the canvas (or your app can flip it onto a window surface).

It’s just a HINSTANCE pointer.

You’re right about the waiting, but that’s entirely app driven. Browsers don’t want to render at 144fps but rather wait until drawing has occurred in order to update the view.

wgpu, dawn, already support drawing to arbitrary surfaces (not just a canvas but any window surface).