XHTML Basics

Modern browsers (IE8, Firefox 3, Opera 9, Chrome, Safari) will display XHTML Strict documents in Standard mode
Attempt to display non-standard documents in Quirks mode
Major difference is width of elements (the cyan strip should be 500 pixels long).
If DOCTYPE is XHTML strict, all use Standard mode
If no DOCTYPE at the head, most use Quirks mode

Bug in IE6: if xml declaration precedes DOCTYPE, IE6 uses Quirks mode!

Strict Doctype, full system URL and Public

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

Strict Doctype, full system only

<!DOCTYPE html "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

Strict Doctype, abbreviated system

<!DOCTYPE html "DTD/xhtml1-strict.dtd">