Here I will explain you how to set the Document Type Definition (DTD) of an XHTML web-page. The DTD tag goes at the start of your coding before the starting html tag. There is no end tag or self-closing for the DTD tag.

There are 3 Doctypes usable by an XHTML document. Strict, Transitional, and Frameset. The doctype is used when validating an XHTML document’s content against a DTD to find markup errors. More >