This is very cool and it arrives at a good time: I have a whole delete-back-word thumb key on my split-key, and I've been trying to train myself to use it with little success. This is actually perfect.

I recognize this might push against the minimalism you're going for, but I might suggest having the next word, or even two words, off to the right and greyed out. What you're measuring is recognizing and typing the word, and most competent typists who want to lower error rates and gain speed are going to be fast enough already that they (we) can be recognizing the next couple of words while typing the one which is already centered.

A two-word lead would let me get up to speed, maybe that should be configurable but for the words I've seen while playing with it, probably two is just right.

Great site, I'll come back and use it as-is.