Why 35 minutes
Thirty-five minutes is an awkward, useful number: a long enough oven roast for most vegetables, and a common length for a school lesson's working portion after registration and setup are taken out. It is the sort of duration you set precisely because the round numbers do not fit.
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 35 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.