FOSDEM: The Wayland display server

> I said this before and will say again: toolkit-level network transparency can be experimented with today, whether X11 or Wayland is the display server, and it should yield superior performance, and have some other goodies: for instance, it could render the GUI according to user's locally defined GTK+ theme rather than whatever pixels the server wants to push (because server doesn't push pixels, it will push statements like 'add a button with these attributes inside that container, please').
That sounds nice in theory, but I doubt it'll work. For example, what if the client uses GTK+ version n, and the server only has version n-1 available?
Also, you'd need to run one server for Qt, one for GTK+, one for EFL etc. etc., that's going to be a nightmare.
What I'd like to see is a rendering server that allows clients to upload little programs that know how to, say, draw a button, and then in order to draw a button, they only have to tell the server to run that program they've uploaded before using such and such parameters. I think DisplayPostscript worked like that. That would at least get rid of the need to run multiple servers for the toolkits, as they could more or less define their own specialized protocol.
However, I'm not sure that'll work either. It would probably require rather substantial changes in the toolkits, and it would probably be hard to port existing theme engines etc. to such a new paradigm, at least if one wants to exploit this kind of technology to a significant degree. Also, we already have similar technology: JavaScript and Canvas/WebGL. GTK+ already features the Broadway backend which renders into a browser window, though I'm not sure how much work is actually done on the client or on the server side.
So clearly, this is a hard problem to solve.