Sub-millisecond accuracy
The timer reads from performance.now(), which on modern browsers is accurate to fractions of a millisecond. Switching tabs does not pause it — when you come back, the displayed time matches actual elapsed time. For official race timing you still want a real chronometer, but for training, intervals, swim sets, and informal events this is plenty.
How to record useful splits
Press Lap every time an athlete, lane, station, or interval marker passes. The first time column shows the split since the previous lap; the second time column shows total elapsed time. That makes it useful for comparing 400 m repeats, swim lengths, cycling segments, or rehearsal runs where you care about both the current split and the full effort.
Before a real session, fullscreen the timer, disable screen sleep, and do one test lap so you know the button placement. If the times matter for a coach or teammate, screenshot the lap list before you reset the page.