Extensible. Ponder the power of this word, for it is the crux of the new Internet model. The next generation Net will be truly extensible, allowing any human creation to be expressed, and accommodating any and all input and output devices that exist or can be devised. A virtual universe infinite in all directions. In other words, if you can dream it up, you can create it, with xml.

http://wdvl.com/Authoring/Languages/XML/Tutorials/Building_Languages/