A DOCTYPE statement (or DTD), located on the first line of an HTML document, designates the standard of HTML which you have chosen to use. HTML Validators require DTD statements. Without a DTD, they will usually fall back to a default DTD (many times HT ML 2.0), most times causing many errors.
Three common DTDs are:
There are many more DTD statements available for use, in the Helpful URLs section you will find a link for a listing of them.