A Document Type Definition (DTD) is a set of rules that define the document structure in a particular version of XML or (X)HTML. A browser will parse the web page looking for the Document Type (DOCTYPE) Declaration in your CSS code and will use these rules to validate the page.
An example of Document Type (DOCTYPE) Declaration is:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The DOCTYPE declaration is included at the start of the (X)HTML document. It is required in order for the page to validate as the browser needs to know which DTD to use in order to process the page correctly.
In the previous example the DTD XHTML 1.0 Transitional is used.
