Five hours is a travel day chunk, a test window, or a slow reduction. Check the device is plugged in. Background tabs stay accurate because the end time is a timestamp, not a count of ticks.
Still, a five hour timer is a bad thing to leave on a laptop that will sleep in a bag. Plug it in and leave the lid open, or use wake lock. 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 5 hours 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 5 hours timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Still, a five hour timer is a bad thing to leave on a laptop that will sleep in a bag. Plug it in and leave the lid open, or use wake lock.
What is 5 hours actually good for?
Five hours is a travel day chunk, a test window, or a slow reduction. Check the device is plugged in.
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.