Can you tell about DTD (Document Type Declaration)?

• An instruction that links a specific XML or SGML document (such as a webpage) with a document type definition (DTD) is known as a document type declaration, or DOCTYPE (for example, the formal definition of a particular version of HTML 2.0 - 4.0)

An instruction known as a document type declaration (DOCTYPE) links a specific XML or SGML document (such as a webpage) with a document type definition (DTD) (for example, the formal definition of a particular version of HTML 2.0 - 4.0). It appears in the document's serialised form as a brief string of markup that follows a certain syntax.