The GDK Broadway backend allows GTK applications to be displayed in a web browser using HTML5 and web sockets. To use this feature, start the broadway server, gtk-broadwayd, and specify the display number as a commandline argument. Then, open your web browser and go to http://127.0.0.1:8085. You can run multiple applications in the same browser window by setting the BROADWAY_DISPLAY environment variable. The default display is 0, and the display number determines the port to use when connecting to a Broadway application.
https://docs.gtk.org/gtk4/broadway.html