Online stopwatch

Start, and it counts up in hundredths. Lap records the time without stopping the clock, and each lap shows the split from the one before it.

Hundredths are shown because that is what a stopwatch is for, but it is worth knowing what that precision actually means here. The display updates once per animation frame — about sixty times a second — and your own reaction time on the button is around two hundred milliseconds, so a hand-timed result is genuinely accurate to roughly a fifth of a second regardless of how many digits appear. This is why hand timing in athletics is conventionally rounded up to the tenth, and why records need electronic timing.