I didn't bother to test in anything besides Chrome on Mac. I was going to put in an audio alarm, but then I'd have to put in a button to disable audio, then I'd have to make it look pretty, and find a good alarm sound.

That feature is on my todo list, as well as email notification, persistent countdowns (you go to /23h and it'll redirect you to a permalink for that timer), email/SMS notification. Next time I have an hour to dedicate to this, I'll them done.

I always have a browser open, so being able to see the countdown status in the titlebar was sufficient for my needs. If I really wanted something to alert me, I'd use my phone in case I step away from my computer.

EDIT: Yeah, looks like Firefox performs awfully with this.

EDIT2: seems to be fixed on FF now.