I'm not sure I understand when you say there can be "objects" and "action entities" in the GUI description language.
When you say "handles", do you mean something like DOM events?
Your description language will include CSS or its equivalent for styling, right?
What will you use for your description language anyway? HTML? Custom XML?
Having to compile the description language files first might be a bit awkward for some people. Some people might want to load the files on the fly, like scripts.
A suggestion: Please make it possible for me to write a custom renderer and resource loader/manager for your GUI so that it can go through my render and resource loader/manager.
Say I have an item sprite. I'd like to show it on the ground in the game world and inside an inventory screen. It would be nice if the GUI framework can use the same sprite resource as what I use when rendering the game world.
Take a look at how libRocket does custom renderers.