XML Schema definition language
I am biting the bullet and diving deep to get a full handle on the XML Schema definition language which defines the rules for constructing rules for how XML documents are constructed. In other words, if you want to define a new XML document format, you need to construct a schema for it. If you want to construct a schema, you need to understand the rules for XML Schema. There are in fact tools to simplify the task of constructing XML schemas, but for my purposes I really do need to become proficient, if not expert, in the finest level of details of XML schemas.
I am starting with the primer, XML Schema Part 0: Primer Second Edition, W3C Recommendation 28 October 2004. This is very dense material, but the primer includes lots of examples.