Web Design in a Nutshell, 3rd Edition

Document Type Definition (DTD)

A Document Type Definition (DTD) is a file associated with SGML and XML documents that defines how markup tags should be interpreted by the application reading the document. The DTD uses SGML syntax to explain precisely which elements and attributes may appear in a document and the context in which they may be used. DTDs were briefly introduced earlier in this chapter. In this section, we’ll take a closer look.

A DTD is a text document that contains a set of rules, formally known as element declarations , attlist (attribute) declarations, and entity declarations. DTDs are most often stored in a separate file (with the .dtd suffix) and shared by multiple documents; however, DTD information can be included inside the XML document as well. Both methods are demonstrated later in this section.

Reading DTDs

While you may never be required to write a DTD, knowing how to read one is a useful skill if you plan on getting cozy with XHTML or any other DTD released by the W3C. This chapter should give you a good start, but you may also want to check out these online resources.

• “How to Read W3C Specs” by J. David Eisenberg at http://www.alistapart.com/articles/readspec/

• W3Schools DTD Tutorial at http://www.w3schools.com/dtd/default.asp