Creating Valid XML
As you have seen, XML validators recognize well-formed XML, and this is very useful for picking up syntax errors in your document. Unfortunately, a well-formed, syntactically-correct XML document may still have semantic errors in it. For example, a job in Listing 16.4 with no location or skills does not make sense, but without these elements, the XML document is still well-formed, but not valid.
What is required is a set of rules or constraints that define what is a valid structure for an XML document. There are two common methods for specifying XML rules—the Document Type Definition (DTD) and schemas.
Document Type Definitions
A DTD provides a template that defines the occurrence, and arrangement of elements and attributes ...