Document Type Declarations

XML documents specify which DTD they use via a document type declaration (also called a DOCTYPE declaration).

When the DTD is an external document, the DOCTYPE declaration identifies the root element for the document, lists the method used to identify the DTD (SYSTEM or PUBLIC), and then finally provides the location or name of the DTD itself. When using an external DTD, it is recommended that you include the standalone attribute set to “no” in the XML declaration.

A SYSTEM identifier points to the DTD file by location (its URI), as shown in this example:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE compilation SYSTEM "http://www.littlechair.com/notreal/comp.dtd">

DTDs that are shared by a large community or are hosted at multiple sites may have a PUBLIC ID that specifies the XML application. When public IDs are used, it is common practice to supply an additional SYSTEM URI because it is better supported. Web developers who write documents in XHTML will be familiar with the following DOCTYPE declaration that indicates the root element (html) and the public identifier for XHTML Strict. This declaration also specifies its URL as a backup method.

<?xml version="1.0" standalone="no?">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

As an alternative, the DTD may be included in the XML document itself, rather than as an external .dtd document. This is done by placing the DTD within square brackets in ...

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