Why 5 minutes
Five minutes is the classic steep for black tea, the standard Pomodoro break, and about the shortest meditation session that still does something. It is the single most-searched timer length there is, largely because it is the default answer to "just give me a few minutes".
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 5 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.