Appendix A. XML DTDs
A DTD (Document Type Definition) provides a detailed description of a particular XML markup language and can be used by XML parsers to verify that XML instances conform to that markup language. For example, the Address Markup Language we created in Chapter 2 can have its own DTD. When a validating parser reads an Address Markup instance, it can use the Address Markup DTD to verify that the instance contains the correct elements and attributes in the right organization.
Web services use the W3C XML Schema Language, not DTDs, so understanding DTDs is not critical to reading this book. You can read this appendix to help round out your understanding of XML, but it's not necessary.
A DTD may be included directly in an XML document ...