Yes, in most browsers. Audio alerts can fire from backgrounded tabs on desktop Chrome, Firefox, Safari, and Edge. Some mobile browsers require a user interaction with the page before allowing audio to autoplay, so if the alarm stays silent, tap the page once after starting the timer.
How accurate is the countdown?
When you start the timer, the end time is captured as a single absolute timestamp. On every animation frame, the remaining time is recomputed as end minus now. That means accuracy is typically within a few milliseconds and is not affected by tab throttling or dropped frames, because the timer reads the clock rather than counting ticks.
Does the timer keep running if I close the tab?
No. Closing or reloading the tab resets the timer. If you need a countdown that survives a reload, bookmark a Days Until page for the specific date, which recomputes from the current moment on every page load.