Why 90 minutes
Ninety minutes is one full sleep cycle, which is why waking on a multiple of ninety often feels better than waking on a round number of hours. It is also the length of a football match, a typical feature-film first act plus second, and about the outer limit of deep focused work for most people.
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 90 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.