Privacy
Nothing you type in a lesson ever leaves your browser.
Where your progress lives
Your lesson text, your keystrokes and your per-key statistics are kept in this browser's own storage and nowhere else. There is no account, no login and no server holding a copy: “Save a backup file” is a download to your own computer. That is also the catch - clearing this browser's data would lose your practice, so a backup now and then is worth the minute it takes.
The one thing you can send
“Report a problem” opens a box. What you write there, plus - unless you untick it - a few numbers about your progress (which language and keyboard you practise, lessons finished, average and best speed, the app version and which browser you use), is sent when you press Send, and not before. Never the lesson text, and never anything you typed in a lesson.
What else is sent
Nothing. This site has no usage statistics and no crash reporting switched on: no request to anybody, no third-party script, no identifier.
There are no cookies, no advertising and no third-party trackers. The service worker that lets the app work offline never touches an analytics request either: not to cache it, not to delay it, not to retry it.