What a DTD Is
A DTD is a description of the allowed structure of a class of XML documents.
In a DTD, you declare elements, attributes, and so on that are allowed in the structure of a corresponding class of XML documents. Elements and other parts of an XML document are declared in markup declarations.
The following are the types of markup declaration in XML 1.0:
Element declarations
Attribute list declarations
Entity declarations
Notation declarations
Markup declarations may be contained in part or entirely within parameter entities.
The DTD Is Not the DOCTYPE Declaration
Earlier, when defining a valid document, this chapter indicated that a valid document complies with the constraints expressed in the DOCTYPE declaration.
It is important to be ...
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