The document type declarations is always the first line before the html tag in a web document.
HTML 4.01 Transitional
Allows depracated html tags. Cannot be used with frames.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
HTML 4.01 Strict
Does not allow deprecated html tags. Cannot be used with frames.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
HTML 4.01 Frameset
Same as HTML 4.01 transitional but allows frames.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
XHTML 1.0 Transitional
Allows deprecated tags. Framesets are not allowed. Markup must be written as well-formed XML. For XHTML doctypes the html tag should be <html xmlns=”http://www.w3.org/1999/xhtml”>.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
XHTML 1.0 Strict
Framesets are not allowed. Markup must be written as well-formed XML.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
Html tags not allowed with Strict Doctypes:
<applet> <basefont> <center> <dir> <font> <isindex> <menu> <s> <strike> <u> <xmp>
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