doctype
The HTML syntax requires a doctype to be specified to ensure that the browser renders the page in standards mode. The doctype has no other purpose.
The doctype declaration for the HTML syntax is <!DOCTYPE html> and is case-insensitive. Doctypes from earlier versions of HTML were longer because the HTML language was SGML-based and therefore required a reference to a DTD. This is no longer the case and the doctype is only needed to enable standards mode for documents written using the HTML syntax. Browsers already do this for <!DOCTYPE html>.
Source: http://www.w3.org/TR/html5-diff/#doctype