About waking at 6:45 AM
Quarter to seven is the compromise alarm: the last point from which a full morning routine still fits before an eight o'clock departure. Set it and there is no slack, which for some people is exactly the right amount of pressure.
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.