Online alarm clock
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Quick alarms
Keep this tab open: the alarm rings even in the background. If you close the browser it won't ring.
Choose the time, press Set alarm and leave the tab open. At that time the alarm rings with the sound you picked, the button flashes and you get a notification if you are in another window.
Use Snooze for five or ten more minutes, or Stop to turn it off. If it is already past that time today, the alarm is set for tomorrow.
The alarm keeps the screen awake on phones and survives a page reload. It only needs the browser to stay open.