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:
Attribute list declarations
Markup declarations may be contained in part or entirely within parameter entities.
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 ...