I am not a fan of this. Browsers by now have awesome text layout and rendering support, highlighting, screen reading, zooming. Individual images can be seen/copied/saved, user scripts and styles and browser plugins can access the loaded html content, and so on.

All this will have to be redone in canvas for no good reason and I doubt it will be nearly as good as what browsers can do already, and we will lose a lot of openness and interop.