Audio / video sync test
A ball drops onto a fixed floor once a second; the click is scheduled on the audio clock, so what you see and hear should land together.
Both audio and visual run off the AudioContext clock, with the browser's reported output latency compensated, so what remains is the room: display processing plus the audio chain downstream. The floor flashes for exactly one frame at impact; a cursor on the ±250 ms ruler shows where the click landed relative to it. Residual uncertainty is about one display frame at the measured refresh. 0.5 / 1 / 2 Hz.
The floor flashes white for one frame as the click leaves the device. Watch where the lime cursor sits on the ruler when you hear it: that offset is the room — display processing plus the audio chain downstream of the browser. Browser output latency is already compensated; residual uncertainty is about one display frame.