About waking at 6:15 AM
Quarter past six is the fifteen minutes people claw back when six turns out to be more than they need. It is also a good target if you are shifting your wake time gradually — fifteen-minute steps every few days move a sleep schedule with far less friction than an hour at once.
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.