The < !Doctype > is use as a declaration is not a tag. The < !Doctype > is use in HTML that stands for hyper text markup langauge.
It is used as an instruction to the web browser that represents the version of the HTML page. It is used before the Html tag.
There are several version of the HTML that are "HTML 3.2, HTML 4.01, XHTML, and HTML5". Except of HTML5 we use the DTD that stands for document type declaration it is basedon SGML. It specifies the rules for markup language.
