Online timer
Set minutes and seconds (or use a preset), hit Start, and when time runs out an alarm sounds and the page title flashes. Keep the tab open to hear the sound.
What an online timer is good for
A countdown timer comes in handy whenever you need a precise time limit: pasta on the stove, tea steeping (3-5 minutes), a study session, circuit training at the gym, rest between sets, or letting hair dye set. Compared to your phone's clock app, it takes two clicks to set — no unlocking your screen — and the 1-to-15-minute presets cover the most common cases.
Why the sound needs the page to stay open
Browsers only allow audio playback in tabs that are active or at least still open: if you close the page, the alarm can't play. Switching to another tab is fine, though — the countdown is based on your system clock, so it stays accurate even when the browser throttles background tabs, and when time runs out the tab title flashes "⏰ Time's up!" so you'll notice the alert even at low volume.