That's a cool idea. I once made a similar site but for a completely different purpose -- language learning. Unfortunately I never finished it but it's still in my TODO list.
The workflow was:
- you read the sentence
- the sentence disappears
- then you type from memory
- every time you mistype something -- the next couple of words appears for a few seconds and you keep typing
That way you not only learn to type that foreign language. But you also memorise the sentence in your short-time memory and you get a feel for sentences are created (order of words, phrases, etc.).
https://github.com/tkrajina/git-plus
Disclaimer, I'm the author ^^