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http://www.w3schools.com/tags/tag_doctype.asp good place to learn HTML, CSS, and other things about web pages.
HTML tag
Definition and Usage
The declaration is the very first thing in your document, before the tag. This tag tells the browser which HTML or XHTML specification the document uses.
HTML
HTML 4.01 specifies three document types: Strict, Transitional, and Frameset.
HTML Strict DTD
Use this when you want clean markup, free of presentational clutter. Use this together with Cascading Style Sheets (CSS):
HTML Transitional DTD
The Transitional DTD includes presentation attributes and elements that W3C expects to move to a style sheet. Use this when you need to use HTML's presentational features because your readers don't have browsers that support Cascading Style Sheets (CSS):
Frameset DTD
The Frameset DTD should be used for documents with frames. The Frameset DTD is equal to the Transitional DTD except for the frameset element replaces the body element:
XHTML
XHTML 1.0 specifies three XML document types: Strict, Transitional, and Frameset.
XHTML Strict DTD
Use this DTD when you want clean markup, free of presentational clutter. Use this together with Cascading Style Sheets (CSS):
XHTML Transitional DTD
Use this DTD when you need to use XHTML's presentational features because your readers don't have browsers that support Cascading Style Sheets (CSS):
XHTML Frameset DTD
Use this DTD when you want to use frames!
To check that you have written a valid XHTML document with a correct DTD, you can link your XHTML page to an XHTML validator.
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