Well, you can ofcourse support more advanced features, but it means that for GUI-frameworks that do not natively support such a feature the author of "AnyGUI" would have have implement this using the capabilities that are available.
This is however obviously a rather tedious job and makes AnyGUI a very complex project.
This sounds a bit like the approach taken by wxWindows. It interfaces to GTK, motif, the Windows API, and OS X (I think). Anything that one or the other can't do is implemented directly by wxWindows. So everybody gets a high-functioning almost native toolkit. Pretty sweet.
Too bad I haven't been able to get wxPerl to compile, but that's just me :-)
I just realized that I was using the same sig for nearly three years.