About waking at 9:00 PM
Nine at night is a wind-down alarm — screens off, lights down, start the run-in to sleep. Setting an alarm for bedtime is unglamorous and works considerably better than deciding to go to bed earlier.
Leave the tab open
A browser alarm can only ring while its page is open. It does not have to be the tab you are looking at, and on a desktop it does not have to be the window in front — but closing the tab or quitting the browser stops it. For anything you genuinely cannot miss, set a second alarm on your phone as a backup. That is true of every browser-based alarm clock, not just this one.