Is there a way to start HTML5 games in fullscreen by default when they’re loaded without needing to press the fullscreen buttom?
No, it’s impossible because of security reason.
You can however make the canvas fit the entire browser window.
How would I go about doing this?
Here is a demo from a while ago doing this, you can see the CSS there https://www.pixcade.com/cosmos/web/
It’s a very old Defold engine version now and inefficient function use so will be slow on mobile but that’s unrelated to fit window stuff. I think I added some stuff to fix mobile fit window stretching but can’t remember at the moment.