Presets:
Set the time, press Start, and an alarm sounds when it reaches zero. The remaining time is also shown in the browser tab title, so you can keep working in another tab. The timer is based on the clock, not on a tick count, so it stays accurate even if the tab is in the background. Keep the page open until it finishes: it cannot ring after you close it.
The Pomodoro technique alternates 25 minutes of focused work with a 5 minute break. Use the 25 min preset, then the 5 min preset for the break.
Usually yes, but some browsers block sound until you have interacted with the page. Pressing Start counts as that interaction.
No. The timer runs entirely in your browser.
See also: Stopwatch · Pace Calculator · Hours Calculator