Setting the time
Use the + and − buttons, type into the boxes, or press the arrow keys in a box. Seconds over 59 are carried into minutes when you leave the box. The presets set a whole number of minutes in one tap. While it runs, +1 min adds a minute without stopping it.
When it reaches zero
A chime plays, made in the page with your browser's audio, the ring pulses gold, and the tab's title flashes "Time's up" so you can spot it from another tab. Turn Sound off for a silent timer. With Repeat on, it chimes and starts the same time again straight away, for anything you do in rounds.
A link that sets the time
The address follows the time you set: /timer#5m, /timer#1h30m or /timer#90s. Bookmark one or send it to someone, and the timer opens with that time ready. Copy link copies it for you.
In a background tab, or asleep
The timer keeps the moment it will end and works out what is left from your device's clock, rather than counting ticks, so it stays right in a background tab. Browsers slow down hidden pages, so the timer checks the clock from a small background worker that browsers leave alone, and the chime comes within a second of zero. While it runs it asks the browser to keep the screen on; if the device sleeps anyway, the timer catches up from the system clock when it wakes and chimes then if the time ran out.
Questions
Why did I not hear the chime?
Check that Sound is on and the device is not muted. Browsers only let a page make sound after you have pressed something on it, which pressing Start does; a timer started from a link still needs that press.
Can I run two timers?
Yes. Open the timer in a second tab. Each tab keeps its own time, and each tab's title shows its time left.
Is anything saved?
No. The time lives in the page and in the address, and nothing is stored on your device or sent anywhere.