HTML <!DOCTYPE> Declaration
--
Definition and Usage
The doctype declaration should be the very first thing in an 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 the markup language the page is written in.
The doctype declaration refers to a Document Type Defination(DTD). The DTD specifies the rules for the markup language, so that the browsers render the content correctly.
--
Doctype Avalable in the W3C Recommendations
XHTML 1.1
This DTD is equal to XHTML 1.0 Strict, but allows you to add modules (for examp.e to provide ruby support for East-Asian languages).
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
--
http://www.w3schools.com/tags/tag_doctype.asp