These documents provide a good starting place for learning about XML.
[XML-Intro] TEI, ed. “A Gentle Introduction to XML.” Text Encoding Initiative (TEI), March 2002, http://www.tei-c.org/Guidelines/P4/html/SG.html.
[XML-Tech] “A Technical Introduction to XML.” February 1998, http://nwalsh.com/docs/articles/xml/..
[RNG-Intro] “RELAX NG Tutorial.” OASIS Open, December 2001, http://www.relaxng.org/tutorial.html..