I'm not sure I'm convinced yet, but it turned out to be fairly straightforward to hook up libRocket with Allegro 5. Sample program attached.

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Thats cool. Does libRocket work best with programs that use opengl?

I haven't looked at the rocket documentation much, but can you explain what RenderGeometry is doing in your example program? What are those vertices?

It should work with D3D just fine. I didn't test.

Rocket sends the things it wants to draw, and RenderGeometry is responsible for getting that onto the screen.

I wonder how it works within a real game... is it fast enough? Can you have elements randomly dispersed on the screen? Etc.