HTML And XHTML Doctypes
In order to validate any page of HTML or XHTML you will need a doctype. This is a string of text that sits at the top of the document and tells the browser exactly what markup standard has been used to create the page.
XHTML Strict
This doctype is used in an XHTML document when you are not using any framset or depreciated tags.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
XHTML Transitional
This doctype is used if your XHTML document contains depreciated tags like .<.p>