The DOCTYPE
declaration is needed if the web page needs to be validated against
a particular DTD. HTML validation is done if the DOCTYPE is specified.
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Each
browser has two types mode for rendering the page. One would be the
standard mode and the other would be the quirks mode. In the standard
mode the web page is validated against the DTD specified in the DOCTYPE
declaration.
When you
specify the DOCTYPE, the html page is validated against the specified
standard and found out if it is valid or not. So what will happen if
you have not specified that? The browser will go into the quirks mode
where validation is bypassed.
There are
many valid DOCTYPE declarations that can be used in an html page. You
can find this list in the URL
http://www.w3.org/QA/2002/04/valid-dtd-list.html