Time anything to the hundredth of a second, record laps, and go fullscreen for a big, clear display.
Space start/pause · L lap · R reset · F fullscreen
It measures against your device's clock rather than counting ticks, so it doesn't drift when your browser is busy or the tab is in the background. The display refreshes about 25 times a second and shows hundredths of a second; the real limit is the accuracy of your device's own clock.
Yes. Because it works from the clock, the time is still correct when you come back. While it runs, the page asks your device to keep the screen awake where the browser allows it, and the current run is saved on your device so an accidental reload picks up where you left off.
Press Lap while the stopwatch is running. Each lap shows the time since the previous lap, plus the total so far. Once you have two or more different laps, the fastest is marked green and the slowest amber. "Copy laps" puts the list on your clipboard.
Press the expand icon at the top right of the stopwatch, or the F key. Press Escape or the icon again to leave. On phones whose browsers don't offer true fullscreen (such as iPhone), it fills the whole page instead.
No. The only thing saved is the current run, in your own browser's storage, so it survives a reload. Reset clears it.