For Firefox there are a couple of other solutions for this problem, but for Chrome I have not been able to find any.
This is particularly an issue as these solutions also disable the first, related popup that asks you whether you want to allow or deny the website to put you in fullscreen, not just the notification shown after allowing a site to go into fullscreen.
1: Via about:config
- Type in
about:config into the navigation bar and press enter.
- Search for the value
full-screen-api.approval-required and set it to false.
2: Via a plugin
Install the Firefox plugin Disable HTML Fullscreen Alert.