In Part 1 of the book, you learned XML’s underlying grammar, which forms the rules for writing an XML document. In Part 2, you learned how to transform an XML document into another form, in this case HTML. Now, in Part 3, you’ll learn how to define a custom markup language in XML.
To define such a language, you will first identify its elements and their attributes, declaring which are required, and which are not. This information is called a schema. For example, an historian might create WowML, the (fictitious) Wonders of the World Markup Language, as a system for cataloging data about the wonders of the world. WowML might have elements like
Schemas, while not required, are exceptionally ...