The doctype declaration is used to specify what version of HTML or XHTML is being used, and provides the W3C address for browsers to download the 'document type declaration (DTD)'. Without this tag browsers may not be able to correctly display your page. Most webmasters prefer to work with HTML 4.01 transitional because this gives the most compatibility with older HTML tags. However, this compatibility comes at a price and these webmasters often have to accommodate the quirks of different browsers so their pages all look the same from browser to browser.
If strict compliance with standards is required, which allows webmasters to focus on content and design instead of browser quirks, then selecting a strict doctype is advisable. The page will need to comply with the selected standard.