From an interview with James Clark, entitled:
JC: I knew how insanely complex writing an SGML parser was. SGML is really doing something very simple. It's providing a standard way to represent a tree, and your nodes have a label with names and they can have attributes. That's all it's doing. It's not a complicated concept.
Of course, that's really MicroXML, not XML.[Personally, I wish people would stop fueling this notion of XML being too complex.]
James wouldn't agree: see his manifesto at http://blog.jclark.com/2010/12/microxml.html and the MicroXML CG draft at https://dvcs.w3.org/hg/microxml/raw-file/tip/spec/microxml.html.
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GMail doesn't have rotating .sigs, but you can see mine at http://www.ccil.org/~cowan/signatures