For security reasons we have to serve any code uploaded by users within an iframe. I’m not familiar with the nuances of PWAs with iframes. You may have to find a work around to get it to work.
I suppose itch.io could provide some kind of global system system for doing PWAs for html5 games. We tend to avoid having direct links to games made available though since we want people to interact with the itch.io UI when they play the game. So I’m not sure how this integration might work.