I love Knuth's books for the fact that they teach you advanced algorithm features 'from the ground up', as it were - it's great to see someone else make an emulator too.
Unless I'm not understanding the UI, it needs a "run" button so I don't wear out a mouse button mashing "step". Yes?
Please also add a MMIX version and finally translate all the MIX code from Knuth's "Art of Computer Programming" to MMIX ;)
It's great this also works on mobile devices, but it would be better if the "step" and "back" buttons were more touch friendly (bigger).