Stopwatch

Yes, closing the browser tab will reset the stopwatch. Keep the tab open during timing.

There is no limit on lap time recordings. All laps are displayed in a list showing both split and cumulative times.

Yes, because the start moment is stored and every repaint simply compares it with the current clock. Browsers throttle timer callbacks in background tabs to save battery, so the display may refresh less often, but the accumulated total does not drift. Note, though, that closing a laptop lid puts the machine to sleep while the system clock keeps running, so the elapsed time will include that period once it wakes.

A lap time measures the interval since the previous mark; a split time measures the total from the start up to that point. Run three laps of a 400 m track in 90, 95 and 92 seconds and those are your lap times, while the splits read 90, 185 and 277 seconds. Read laps when you want to see whether your pace is wobbling between segments, and splits when you need to know how much room is left against a target finish.