What is stored
Three things, all in this browser’s local storage: your colour-theme preference, any custom palette you build, and your drill record — which levels you have run, how you answered, and which pairs you confuse. Nothing is transmitted anywhere, because there is nowhere for it to go: this site has no accounts, no login, and no backend.
What is not
No analytics, no advertising, no third-party scripts, no embedded players, and no fingerprinting. The site loads its own stylesheet, its own fonts, and its own modules, and its content security policy is written to refuse anything else.
Deleting it
Clear site data for this domain in your browser settings and all of it is gone, including your progress history. There is no copy elsewhere to request the deletion of. Using a different browser or a private window starts from nothing by definition.
Server logs
Requests are served through Cloudflare, which keeps standard operational request logs. Those are infrastructure records — not a profile of your practice — and nothing in them is joined to your drill history, which never leaves your device.