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Can someone explain to me the advantages of using a DTD (Document Type Declaration)?<br />
Do I need to go back to all of my old web pages and add this?<br />
What is it supposed to do FOR my web pages or TO the web browser?<br />
What happens if I don't have one?<br />
Which numeric version should I use?<br />
When I try to verify HTML at web sites, I am frequently told that the web page has problems even though it works, why?<br />

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Can someone explain to me the advantages of using a DTD (Document Type Declaration)?
Do I need to go back to all of my old web pages and add this?
What is it supposed to do FOR my web pages or TO the web browser?
What happens if I don't have one?
Which numeric version should I use?
When I try to verify HTML at web sites, I am frequently told that the web page has problems even though it works, why?


Need explanation on why I need a DTD

Web development can range from developing the simplest static single page of plain text to the most complex web-based internet applications, electronic businesses, and social network services using a wide variety of languages and standards, including the familiar HTML, JavaScript and jQuery, ASP and ASP.NET, PHP, ColdFusion, CSS, PHP, Flex and Flash, but also the implementation of a broad list of standards including XML, WSDL, SSDL, VoiceXML and many more.

Web Languages and Standards

Web development includes all aspects of presenting content on intranets and the Internet, including delivery development, protocols, languages and standards, server software, browser clients, databases and multimedia generation.