itch.io

It all works on Windows too cause the magic of opensource. Almost every foss browser does. No probs here.

It is just I wouldn't disregard the possibility that Unity or Itch use not fully compliant methods to cut development time or whatever. In this case some settings/HTML shell/Javascript tweaking might be needed.
Unfortunately I haven't tried to make a mousegrab in my Godot to test things so i can't tell what exactly might be going on under the hood.