Some HTML5 games like those made in Godot have trouble running when directly opened locally.
Right now, the itch.io client seems to just download the files and extract them to a directory then opens it directly from there. My suggestion is, when the user tries to launch a downloaded HTML game, spin up a local web server in that game's folder and load the path
in the pop out window instead. Of course when the game is closed, the local web server should be terminated as well.