About waking at 10:00 AM
Ten in the morning is usually a reminder rather than a wake-up: a standing meeting, a medication, or a market opening. Setting it as an alarm rather than a calendar entry works better for things that repeat daily and do not need a calendar invitation.
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.