DOCTYPES
OR
<!doctype html>
<html lang="en-us">
The above is the new HTML5 DOCTYPE recommendation.
Definition and Usage
The 'doctype' declaration is not an HTML tag; it is an instruction to the web Browser about what version of the markup language used to create the web page.
The <!DOCTYPE> declaration must be the very first thing in your HTML5 documents and before the <html> tag. This tag tells the Browser to which HTML specification the document adheres. It is important that you specify the 'doctype' in all your HTML documents.
The 'doctype' in HTML 4.01 required a reference to a DTD (Document Type Defination), because HTML 4.01 was based on SGML (Standard Generalized Markp Language). HTML5 is not based on SGML and therefore does not require a reference to a DTD, however, it does need the 'doctype' statement for Browsers to behave as they should.