Xpra (see https://github.com/Xpra-org/xpra-html5) is very similar to Guacomole, except the backend Xpra server is written in Python (and Cython), whereas Guacomole's server is written in Java and C. Xpra native frontends, but also an html5 frontend that you can embed in web applications. For example, CoCalc provides a modified version of that html5 frontend to serve graphical X11 apps: https://cocalc.com/features/x11