Why 10 minutes
Ten minutes is the standard oven preheat, a beginner meditation session, and the length of a proper break between meetings that almost never gets scheduled. It is also long enough to leave the room, which is exactly why you want an audible alarm rather than a glance at a clock.
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 10 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.