Press START (or the space bar) to begin timing and STOP to pause. Press START again to carry on from where you stopped.
While it runs, press LAP (or L) to record a lap. Each lap shows the time since the previous one and the total time, newest first.
The stopwatch keeps exact time even when the tab is in the background, and it keeps running if you reload the page. Press RESET (or R) when it is stopped to start again from zero.