Do Web developers have to include a DTD when they use XML to describe data?
No. XML can be used to describe data with or without a DTD. The term “valid” XML refers to XML data that references a DTD, while “well-formed” XML refers to XML that does not use a DTD. The addition of “well-formed” XML is one of the fundamental differences between XML and Standard Generalized Markup Language (SGML). Clearly, in both cases, the XML itself must conform to the standards for the language (for example, all tags must be closed and tags cannot overlap).
