Why 20 minutes
Twenty minutes is the sweet spot for a nap: long enough to recover alertness, short enough to stay out of deep slow-wave sleep and avoid waking up feeling worse than before. Sleep researchers are unusually consistent on this number. It is also a common alternative Pomodoro length.
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 20 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.