The document type declaration tag is first tag in your in your HTML document, before the other html tags. It is not an HTML tag; it is an instruction that gives to the web browser about which version of HTML the page we are using. DOCTYPE Declarations In HTML 5 version<!DOCTYPE html> In HTML 4.01 version This DTD consist all HTML tags. but no include deprecated tag for example font. Frames are not allowed in this version. <!DOCTYPE HTML PUBLIC "-//Type here.../DTD HTML 4.01//EN" >