Which DTD Statement To Use?
There is not just one type of HTML, there are actually many: HTML 4.01 Strict, HTML 4.01 Transitional, XHTML 1.0 Strict, and many more. All these HTML types are defined in their respective W3C specifications, but they are also defined in a machine-readable language specifying the legal structure, elements and attributes of a type of HTML.
Such a definition is called a "Document Type Definition"( DTD).
Tools which process HTML documents, such as Web browsers, need to know which DTD an (X)HTML document is actually using: this is why each (X)HTML document needs, at the beginning, a DTD declaration, such as ::
Given its syntax, the DTD declaration is often called, for short, "Doctype".
Why specify a doctype? Because it defines which version of (X)HTML your document is actually using, and this is a critical piece of information needed by browsers or other tools processing the document such as search engine spiders.
But the most important thing is that with most families of browsers, a doctype declaration will make a lot of guessing unnecessary, and will thus trigger a "standard" parsing mode, where the understanding (and, as a result, the display) of the document is not only faster, it is also consistent and free of any bad surprise that documents without doctype will create.
HTML 4.01 - Strict, Transitional, Frameset ::
XHTML 1.0 - Strict, Transitional, Frameset ::
For more information on DTD and Doctype statements visit W3c.org
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