Set it, throw it on the projector, and get on with the lesson. Huge digits, a soft chime at zero, and nothing else on the screen.
Pick a length, press Start, press Fullscreen. The digits fill the screen, the bar underneath drains as the time goes, and at zero the display flashes and a short chime plays. Esc leaves fullscreen. The space bar starts and pauses.
Stopwatch mode counts up instead, with the same giant readout, for anything you want to time without a fixed limit.
Switching tabs does not slow the timer down. Browsers throttle background tabs, so a timer built on counting seconds drifts and finishes late. This one records the clock time it should end at and compares against the real clock every frame, so it ends when it should no matter what else you are doing.
The chime needs one click first. Browsers only allow sound after you interact with a page, so the tone is prepared when you press Start. The sound is generated on this device, not downloaded, which is why it still works with no connection.
You can link straight to a length. Add the minutes to the address: /countdown-timer/?min=5 loads a five minute timer ready to go, and /countdown-timer/?min=5&sec=30 adds seconds. Bookmark the ones you use every day.
It works offline once loaded. After the page has opened once, nothing else needs to be fetched, so a flaky classroom connection cannot interrupt it.
You don't have to take our word for it. Every browser has a built-in panel that shows everything a page sends or receives over the internet. On a computer (this doesn't work on a phone):