Keeps time in the background
Every timer here works out the time from the clock rather than counting ticks, so it stays right in a background tab and catches up after your computer sleeps. The time left shows in the tab's title while it runs.
Questions
Will it make a sound?
The countdown, Pomodoro and interval timers chime when a phase ends, made in the page with no sound files. Turn your volume up, and keep the tab open.