A new tab isn't standard ? I have 12 + itch.io Webgl games on my profile and none of them needed to be opened in a new tab. Making me open a new tab to play your game and my whole screen is just your view box is still basically Fullscreen.
Viewing post in Shadowtail jam comments
https://alexc1991.itch.io/pothole-dodge-returns
This is an unfinished project I did for a game a week for school.
It runs in Godot and developed to be used for the browser for mobile phones and for desktop usage.