What a 20 seconds timer is for
Twenty seconds is how long the CDC recommends scrubbing hands with soap, and the work phase of a Tabata round. The page loads with the countdown already set, so the whole interaction is one click.
The timer runs on the browser clock, not a server, so it works offline once the page is open and it keeps counting in a background tab. At zero it plays a short chime; keep the tab unmuted if you plan to walk away.