The <!DOCTYPE> declaration is the very first thing in your document, before the <html> tag. This tag tells the browser which HTML or XHTML specification the document uses.
Note: The <!DOCTYPE> tag does not have an end tag!
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):
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
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):
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
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):
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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):
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
XHTML Frameset DTD
Use this DTD when you want to use frames!
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
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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