AFAIK it supports contentEditable, but just doesn't show the keyboard for those elements. So theoretically formatting commands and such work.

Alright, but shouldn't the solution be to allow the user to use either Google's keyboard or the built-in(by using textarea)?

The built-in one has a dictionary and some autocompletion, which could be good or annoying... :)

I have played with making an absolutely positioned single-char text input box in my custom editor (https://github.com/dnewcome/richie). The idea is that it forces the built-in keyboard to show but the input is caught and inserted into the DOM. I don't use contenteditable since it doesn't work on most mobile devices. My hacks are really rough but I think the technique could be polished eventually.