Why 2 minutes
Two minutes is the brushing time every dental association recommends and almost nobody achieves unaided; studies of unassisted brushing put the average nearer forty-five seconds. It is also the steeping time for most green teas, which turn bitter if you treat them like black tea and walk away.
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 2 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.