Play along on the drums.
Notes fall down four lanes; you hit each one as it reaches the line. Hold notes are held. Miss the timing and you lose the streak. That’s the whole game.
It plays three ways — the keyboard, fingers on a phone or tablet, or a MIDI drum pad plugged in over USB, which is what it was built for. There’s a calibration step on first run, because the honest latency of your audio hardware is a real number and guessing it makes every note feel slightly wrong.
Bring your own song. Nothing is bundled beyond a demo chart: import a MIDI file and attach an audio track from your own machine, and it builds a playable chart from it. Your files stay in your browser — there is no server, no account, and nothing is uploaded.
Runs entirely in your browser. Headphones recommended.