What is the DTD schema?

A Document Type Definition (DTD) is a set of markup declarations that define a document type for an SGML-family markup language (SGML, XML, HTML).

A DTD uses a terse formal syntax that declares precisely which elements and references may appear where in the document of the particular type, and what the elements' contents and attributes are. A DTD can also declare entities which may be used in the instance document.