Why 45 minutes
Forty-five minutes is a half of football, a university teaching period in much of Europe, and a typical washing-machine cycle. It is the longest interval most people will sustain single-task focus on without a break, which is why so many institutions independently landed on 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 45 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.