The stopwatch uses the browser's high-resolution Performance API, which has sub-millisecond resolution on most modern devices. The display rounds to 10 milliseconds so that it stays readable while running, but the underlying measurement is tighter than that.
Does it keep running if I switch tabs?
Yes. Elapsed time is computed from absolute timestamps captured on start and at each tick, not from an interval counter. So even if the tab is backgrounded, your computer sleeps briefly, or the browser throttles the tab, the displayed time stays correct when the tab becomes active again.
Can I save my laps?
Laps are kept in memory for the current session and shown in a list while the tab is open. Reloading or closing the page clears them. There is no built-in export, but you can copy the lap list out of the page before reloading.