The 2D example does not work in Firefox at the moment, that's sad.
The quake viewer in the html5rocks article does work in FF though.
And it indeed locks the mouse's cursor out of the boundaries in full screen which is what I'd want
Not in "not-fullscreen" though. (EDIT: after test, it's only in FF for now that the cursor is not locked in "not-fullscreen")
It could be indeed an interesting behavior or addition to the mouse plugin once the API is a bit more stabilized and Ashley has more time on his hands.
EDIT: With parallax I can see some 2D FPS and TPS that would still be nice looking and interesting to play.
You'd mostly move in a lateral way, hardly in depth even though it would be theoretically possible (cf CubeMaze by FrenchYann).