Twenty seconds is a Tabata work or rest interval, a hand-washing count, or a 'hold still' for a photo. The interval timer is better for a whole Tabata. This page is one twenty second countdown.
Stand where you can see it. Twenty seconds away from the screen is the whole timer. 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 20 seconds 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 20 seconds timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Stand where you can see it. Twenty seconds away from the screen is the whole timer.
What is 20 seconds actually good for?
Twenty seconds is a Tabata work or rest interval, a hand-washing count, or a 'hold still' for a photo.
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.