Fifty-one minutes is three seventeens: gather, make, review. The gather part has a limit, which stops setup from eating the making.
Review means look at the thing, not open a new tab 'for reference' that becomes the rest of the hour. Press space to start or pause, R to reset, and F for fullscreen. The screen can stay awake after you start, using the wake lock your browser allows. Pick the alarm before you begin. A bell, a chime, a beep, or a short digital pulse all come from the Web Audio API, so there is no sound file to wait on.
This page is the 51 minutes countdown and nothing else is running underneath it. Related lengths are linked under the guide if you want a neighbor, and the timer itself stays above the fold.
Questions
How do I run a 51 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Review means look at the thing, not open a new tab 'for reference' that becomes the rest of the hour.
What is 51 minutes actually good for?
Fifty-one minutes is three seventeens: gather, make, review.
Will the countdown drift if I switch tabs?
No. The end is a timestamp. The display is the gap between that timestamp and now, so a background tab does not lose seconds the way a ticking counter would.