Username or EmailPassword
There's no JS involved (or very little). The "web" version is actually a remote version. The app runs on a server/workstation somewhere. You use the web page to connect to the remote system via a web socket. And then the output is shown in the browser.
This is just using the "web" interface in GTK+ 3.
There's nothing "web" enabled about it. It's really no different from running a remote X11 app.