(Not OP, but I did read the information they wrote.)
> What's wrong with TextEdit on macOS?
Nothing wrong as such, but does it match the “cross-platform” description?
> Firefox … the open/save button does nothing
As per the readme, it is using an API that is not yet supported in Firefox for local file access: https://caniuse.com/native-filesystem-api
> there is no "top menu" or install button
That is meaning the browser's menu. Perhaps there is better terminology that should be used here. The readme does show a screenshot of it, so it seems clear from that to me. The install option is found there.
> and I guess it doesn't work if I don't have an internet connection.
Also in the readme: “Installable as a PWA”, which I think implies offline support, given it doesn't have sync features so has no reason once installed that way to talk to the wider network.