One button, one number, one device. Presses are counted here
and nowhere else: there is no leaderboard, no account and no undo. The line
underneath gets slightly stranger the more different days you come back,
which is either a feature or a warning.
The rest of the family is that way.
How it works
The button is a real button: Tab reaches it, Enter and Space
press it. Storage holds two things — the number and the list of days you
turned up — inside a try/catch, so a browser with storage blocked counts
presses until the page closes and remembers nothing.