About waking at 6:00 PM
Six in the evening is a dinner, gym or collection reminder. It is also a sensible last-caffeine cutoff marker: caffeine has a half-life of around five hours, so an afternoon coffee is still measurably active at bedtime.
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.