Lights Out

Five red lights come on, one by one. Then they all go out together, after a wait you can't predict. Tap the moment they go dark. Go before that and it's a jump start.

Honest note: your screen, browser and touch or Bluetooth input all add their own delay, often a few tens of milliseconds. So beat your own best on the same device, rather than trusting any number you see online, including ours.