Unity WebGL has an issue that makes index.html files completely unplayable under certain circumstances, with the error "both async and sync fetching of the wasm failed". fixes for hosting it on your own server are detailed in this unity forum thread
the issue with itch, however, is that when I zip the build folder, the option to make the file playable in browser is completely absent. I think the issues might share a cause, but I thought I might as well at least ask here.