From RFC 2518 (WebDAV);
XML has been chosen because it is a flexible, self-describing, structured data format that supports rich schema definitions, and because of its support for multiple character sets. XML’s self- describing nature allows any property’s value to be extended by adding new elements. Older clients will not break when they encounter extensions because they will still have the data specified in the original schema and will ignore elements they do not understand.
Hey, I wonder where it says that in the XML Rec? 8-)
See also; a presentation of mine which talks about self-descriptively extensible XML.