Why 30 minutes
Half an hour is the default meeting slot, a standard gym circuit, and the length of most sitcom episodes including advertising. It is long enough to do real work in and short enough to hold attention without a break, which is why it appears everywhere in scheduling.
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 30 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.