A key feature of XML markup is that you validate it. The DocBook schema is a precise description of valid nesting: the order of elements, and their content. All DocBook documents must conform to this description or they are not DocBook documents (by definition). The validation technology that is built into XML is the Document Type Definition or DTD. A validating parser is a program that can read the DTD and a particular document and determine whether the exact nesting and order of elements in the document is valid according to the DTD.
DocBook is now defined by a RELAX NG grammar and Schematron rules, so it is no longer necessary to validate with the DTD. In fact, it isn’t even very valuable since the DTD version doesn’t enforce many DocBook constraints. Instead, an external RELAX NG and Schematron validator must be used.
Validation is performed on a document after it has been parsed. It is possible for parsing errors to occur as well as validation errors (if, for example, your document isn’t well-formed XML). We’re going to assume that your documents are well-formed and not discuss XML parsing errors.
If you are not using a structured editor that can provide guidance on the markup as you type, validation with an external tool is a particularly important step in the document creation process. You cannot expect to get rational results from subsequent processing (such as document publishing) if your documents are not valid.
There are several free ...