Wouldn't it make more logical sense to add canvas to html elements? I.e. checkbox.getContext("2d").fillRect(...)
I guess there are uses for that, but as a web game developer, that wouldn’t really help me. The killer feature is input and text rendering.
Not if you need your html to adapt to the shape of a 3d object drawn in the canvas.
What you described is the CSS painting api (part of CSS Houdini)