Discuss the limits of DTDs, using examples.

Question

What will be an ideal response?

Answer 1

DTDs are commonly used for validation largely because of XML's origins as an offshoot of SGML. SGML was originally designed for text-based documents, such as reports and technical manuals. As long as data content is limited to simple text, DTDs work well for validation. However, today XML is being used for a wider range of document types, and several limitations have prompted XML developers to explore alternatives to DTDs. One complaint about DTDs is their lack of data types. For example, you can declare an age element in a DTD, but you cannot specify that the age element may contain only numbers, or that numbers must fall within a specified range of values. You can declare a dateOfBirth element, but the DTD won't have the tools to force the element to contain only dates. DTDs simply do not provide the control over data that users often require. DTDs also do not recognize namespaces, so they are not well-suited to an environment in which a compound document contains data collected from dozens of XML vocabularies. This is a concern if you are working on several XML vocabularies supporting a wide range of information. Finally, DTDs employ a syntax called "Extended Backus Naur Form (EBNF)," which is different from the syntax used for XML. This means that the document's author must be able to work not only with the syntax of XML, but with EBNF as well. For developers who want to work with only one language, this could be a concern.