⟩ What Is the Relation between XHTML and DTD?
DTD (Document Type Definition) is a language based on XML to you to define a specific type of XML documents.
Since XHTML is a specific type of XML documents, you can use DTD to define the structure of XHTML. In fact, XHTML 1.0 Strict version is defined in DTD as shown at http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd.