The basic functionality is interesting. I think it could use a bit more work, but I also think it would potentially be very useful for large articles. A UI teaching issue so people would understand it, but potentially nice.
Out of curiosity, it works fine for me with the "a" / "s" and "]" / "" keypairs. The "5" / "6" keypair doesn't work. I have a desktop with numeric keyboard, neither the numbers above the letters nor the separate numeric keypad (with num lock on or off) work. Any ideas what might be up?