Why 15 minutes
Fifteen minutes is the quarter-hour that structures most calendars, the length of a decent power nap, and a common short-form Pomodoro for work that is hard to start. It is also the standard interval for checking something in an oven that you do not entirely trust.
It will not drift while you do something else
The countdown is worked out from a fixed end time rather than counted down frame by frame, so putting the tab in the background, locking your phone, or switching apps does not cost you any of the 15 minutes. Browsers throttle background JavaScript to save battery, and that is exactly what makes most browser timers lose time. Press start, go and do the thing you are timing.