Document type declaration

The<!DOCTYPE> is not a HTML tag. It is an instruction to web browser about what version of HTML the page is written in.In HTML 4.01, the<!DOCTYPE> declaration refers to a DTD, because HTML 4.01 based on SGML (Standard Generalized Markup Language). So that the browsers render the content correctly.It is a top level tag known as a Public Text Identifier.HTML5 is not based on SGML so its not require any reference for DTD.For more visit the link.