Twenty-three minutes is a prime block. Do not slice it. One task from start to alarm. It suits a chapter section, a code review of one change, or a drawing you will not perfect.
The useful part is the stop. Mark where you were so the next session starts in seconds, not minutes. 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 23 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 23 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. The useful part is the stop. Mark where you were so the next session starts in seconds, not minutes.
What is 23 minutes actually good for?
Twenty-three minutes is a prime block. Do not slice it. One task from start to alarm.
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.