Setting an alarm
Choose a time, add a label so you know what it is for, and the alarm arms itself. You can set as many as you like and turn each one on or off without deleting it — useful for a recurring set of reminders you only want on certain days. Alarms are checked against the wall clock every second, so one set for 6:00 fires at 6:00 even if the tab has been idle all night.
Leave the tab open
A browser alarm can only ring while its page is open. It does not need to be the tab you are looking at, and on a desktop it does not need to be the window in front — but if you close the tab or quit the browser, nothing will fire. For anything you genuinely cannot miss, set a second alarm on your phone as a backup. This is the honest limitation of every browser-based alarm clock, including this one.
Sound, speech and screen
When an alarm goes off it plays a tone, announces its label out loud, and flashes the page so it is obvious across a room with the sound muted. On a phone the display is kept awake while alarms are armed, so the screen does not go dark and suspend the page.