Pick a ready-made countdown timer, press Start, and an alarm rings when time is up. Each one is a single click away; for any other length use the custom countdown timer.
A 25 minute timer followed by a 5 minute break is the classic Pomodoro rhythm. For longer blocks try the 45 or 50 minute timers.
Yes. They follow the clock and show the remaining time in the tab title, but you must keep the page open for the alarm to ring.
See also: Pomodoro timer · Stopwatch · Countdown timer · All tools