!DOCTYPE
Definition and Usage
The <DOCTYPE>declaration must be the very first thing in your HTML document, before the <html> tag.
The <DOCTYPE> declaration is not an HTML tag; it is an instruction to the 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 was based on SGML. The DTD specifiesthe rules for the markup language, so that the browsers render the content correctly.All HTML need to have a DOCTYPE declared. The DOCTYPE is not actually an element or HTML tag. It lets the browser know how the document should be interpreted, by indicating what version or standard of HTML (or other markup language) is being used.
HTML5 is not based on SGML, and therefore does not require a reference to a DTD.