The DTD makes specific declarations about which elements and attributes may exist in a document and how they should be nested using a brief formal syntax.
A XML language can be defined in a variety of ways. Simple prose, a so-called Schema language, or eventually a combination of the two could be used to specify an XML vocabulary. The XML Standard defines DTDs, the most basic schema language.
The term "Document Type Definition" is used. A DTD is a language made up of rules (also known as a schema) that describes an XML application, also known as an XML vocabulary. For instance, the grammar for the XHTML 1 web markup language is formally defined in the file xhtml1-transitional.dtd, which is accessible through the XHTML 1.0 specification page.
The DTD itself is typically the most crucial component of a DTD-based markup language, but additional constraints can be included as well.
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