Press Start to begin timing and Stop to pause; press Start again to resume. Lap records the total elapsed time and the split since the previous lap, with the fastest and slowest splits marked once you have three or more laps. Timing uses the system clock, so it stays accurate even when the tab is in the background.
The clock is precise to well under a millisecond, but display updates and your reaction time when pressing the button add roughly 0.1 to 0.2 seconds of human error.
Yes. Keep the page open and use Lap for each mile, set or interval. Then use the pace calculator to turn the result into a pace.
No. Everything stays in your browser and disappears when you close the tab.
See also: Countdown Timer · Pace Calculator · Hours Calculator