I'm almost positive we have worked together (your handle stuck out... percussion?)... I agree the extension/validation parts of XML are not well understood (DTD

I'm almost positive we have worked together (your handle stuck out... percussion?)... I agree the extension/validation parts of XML are not well understood (DTD, entity references, namespaces) but in general rarely used or needed. Most uses of XML in the past and today use just mainly tags and attributes. XML purpose was for an extensible document format not a general purpose programming language, DSL and/or a serialization format. For its original purpose it isn't a terrible format. Document formats require special handling of space and comments and repeated nodes (see my other comments). Even S-Expressions ignore spaces in general so are probably not a good format for documents.