The correct procedure for starting a web page in XHTML is as follows:
<?xml version="1.0" encoding = "UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN""\ "http://www.w3c.org/TR/xhtml/DTD/html1-transitional.dtd">
The !DOCTYPE declaration is a top-level tag-like reference known as a Public Text Identifier. It should appear at the very beginning of an HTML/XHTML document in order to identify the content of the document as conforming (theoretically) to a particular HTML DTD specification.
<!DOCTYPE [Top Element] [Availability] "[Registration]// [Organization]// [Type] [Label]// [Language]" "[URL]">