Laps and splits
Press Lap while it runs to close one lap and start the next. Each row shows the lap time, how long that lap took on its own, and the split, the total time on the stopwatch when you pressed it. Once there are two laps or more, the fastest and the slowest are marked. The lap still running is shown under the big numbers.
Keys
Space starts and pauses, L takes a lap and R resets, from anywhere on the page except a text box. Pausing keeps the time and the laps; Reset clears both.
In a background tab, or asleep
The stopwatch does not count ticks. It notes the moment you pressed Start and works the time out from your device's clock whenever it draws, so switching tabs, minimising the window or a busy page cannot make it lose time. If the computer goes to sleep while it runs, it catches up from the system clock when it wakes, so the time includes the sleep, as a stopwatch on a desk would.
Questions
How accurate is it?
It reads a clock that is accurate to well under a millisecond and shows hundredths. The limit is your own reaction time on the button, typically a fifth of a second, so for close finishes take the time from the same person pressing the same key.
How do I save my laps?
Press Copy laps to copy them as a plain table, with the total, ready to paste into a message, a note or a spreadsheet. Nothing is saved by the page itself; closing the tab clears it.
Does it change the tab's title?
Yes, while it runs the tab shows the time to the second, so you can follow it from another tab. The title goes back to normal when you pause or reset.