Why 1 minutes
A minute is the plank hold, the rest between heavy sets, and the length of time a dentist would like you to spend on each quadrant of your mouth. It is also long enough to be surprisingly hard to judge by feel — most people asked to estimate a minute stop somewhere between forty and fifty seconds.
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 1 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.