Forty minutes is a full class activity or a work block with one task and a visible end. It is long enough that you should say what 'done' looks like before you press start.
If attention drops near the end, the task was too vague. Next time, name a product: a paragraph, a problem, a decision. 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 40 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 40 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. If attention drops near the end, the task was too vague. Next time, name a product: a paragraph, a problem, a decision.
What is 40 minutes actually good for?
Forty minutes is a full class activity or a work block with one task and a visible end.
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.