the document TYPE The element specifies the type of document. indicates the type and version of HTML and XHTML that is coded on your document and Document Type Definition (DTD) validates the document. Importantly, all HTML and XHTML document should be validated. This ensures that the code written on your document have no coding errors. View source codes in element: tag must always be placed at the beginning of an HTML document. But when an XML declaration, such as , is present, tag mus be placed after the XML declaration.
Document Type Definition Example