Timer

Yes. When time's up, a sound plays and the tab title flashes "⏰ Time's up!" so you'll notice even if you're not looking at this tab.

Browsers require at least one interaction with the page (like a click) before audio can play. Since you have to click Start, it should play normally in most cases.

Yes. After pausing, clicking Start again continues the countdown from where it left off.

Hours can go up to 23, and minutes/seconds up to 59 each.

No. All calculations happen entirely in your browser and are never sent to or stored on a server.