Croquet - XUL
The idea is that, we could define the Croquet (3d/2d) user interfaces (or the way of filling up the space with content) within a xml file. which satisfies the Croquet software logic requirements (for example: multiplicity of spaces (no direct analogs in VRML or X3D), etc.) and is readable by human.
To try first tiny example in work, just install the two latest XUL and Croquet-Xul packages from http://www.squeaksource.com/MediaTalk.html repository,and add to CroquetMenuProject at any #menuBar.. new menuItem:
menu add: 'Create from xul' target: self action: #actionCreateFromXUL disabled: false.