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Section D.1. What Are DTDs?

D.1. What Are DTDs?

Document Type Definitions express the allowed elements and attributes in a certain document type and constrain the order in which elements must appear within that document type. A DTD is often composed of a single file, which contains declarations defining the element types and attribute lists. (In theory, a DTD may span more than one file; however, the mechanism for including one file inside another—parameter entities—is outside the scope of this appendix.)

D.1.1. Element Type Declarations

An element is the actual instance of the structure as found in an XML document, whereas the element type defines the element, giving it a name and a structure. The form of an element type declaration is:


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