Why most browser timers drift
Browsers throttle background tabs to save battery, cutting timer callbacks to once a second or slower. A countdown that decrements a counter on each tick therefore falls behind — sometimes by minutes over an hour.
The fix is to store the target time and compute the remainder from the clock on each render, so however often the tab is allowed to run, the number shown is right. That is what happens here, which is why switching tabs does not cost you time.
The alarm needs permission first
Browsers block audio until you have interacted with the page, so the sound is armed when you start the timer rather than at the moment it fires. If a countdown finishes silently, that interaction is what was missing.
A tab that is fully discarded by the operating system cannot make any noise at all. For something you must not miss, use a device alarm rather than a web page.
Frequently asked questions
Will it stay accurate if I switch tabs?
Yes. The finish time is stored and the remainder is worked out from the clock, so background throttling does not cause drift.
Why did the alarm not sound?
Browsers block audio until you interact with the page, and a discarded tab cannot play anything. Start the timer with a click, and use a device alarm for anything critical.
Does it keep running if I close the tab?
No. Everything lives in the page, so closing it ends the timer.
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