Why 75 minutes
Seventy-five minutes is a common length for a yoga or barre class, and about as long as most adults can work on something demanding before the returns fall off sharply. It is also roughly the length of a compact-disc side, which is why so many albums stop just short of it.
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 75 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.