The new API confuses me because I can't see what it can really do that you couldn't already achieve by adding some position:absolute elements that float above the canvas. I mean, yes, since it's rendered as a texture, you can apply an arbitrary visual transform… but apparently the input handling is outside your control, so what's even the point? You won't be able to interact with the element properly. I also assume you can't even really occlude things properly? It doesn't make any sense to me.
As a point of comparison: Win32 has APIs that let you draw things like buttons and text input fields without having to instantiate windows (~= DOM elements) for them. Though I'm not quite sure what you can do in the way of input handling without having an actual window. Regardless, browsers manage to contort native controls into funny shapes on behalf of web pages, so the API there definitely offers more possibilities.
As a point of comparison: Win32 has APIs that let you draw things like buttons and text input fields without having to instantiate windows (~= DOM elements) for them. Though I'm not quite sure what you can do in the way of input handling without having an actual window. Regardless, browsers manage to contort native controls into funny shapes on behalf of web pages, so the API there definitely offers more possibilities.