About waking at 7:45 AM
Quarter to eight is the alarm of someone who works close to home or starts late. It is also the last practical wake time before a nine o'clock start becomes a rush, so it tends to be self-correcting after a few late arrivals.
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.