Thirty-three minutes is three elevens. Teach, practice, check. It is a slightly generous half hour for a task with setup you cannot skip.
Write the three labels on a card before you start so the middle does not become a second introduction. 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 33 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 33 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Write the three labels on a card before you start so the middle does not become a second introduction.
What is 33 minutes actually good for?
Thirty-three minutes is three elevens. Teach, practice, check.
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.