Web Design in a Nutshell, 3rd Edition

Document Type Declaration

The example also includes a document type (DOCTYPE) declaration.

 <!DOCTYPE accounts SYSTEM "simple.dtd">

The purpose of the DOCTYPE declaration is to refer to the DTD against which the document should be compared for validity. The declaration identifies the root element of the document (accounts, in the example). It also provides a pointer to the DTD itself. DOCTYPE declarations are discussed in the "DTD Syntax" section later in this chapter and again in Chapter 9 as they apply to XHTML.

Together, the XML declaration and DOCTYPE are often referred to as the document prolog . For XML languages that don’t use DTDs, the entire prolog is optional. For languages with DTDs, the DOCTYPE declaration is required for the document to validate.