The DOCTYPE begins an SGML document and tells the parser which DocumentTypeDefinition, or DTD, to use in checking the document's syntax.

Several web browsers (ab)use the doctype (or the absence of one) to determine how to handle HTML. They go into a "QuirksMode" under certain circumstances. This is for backwards-compatibility with web pages not designed to the standard.

Common DTDs

HTML 4.01

Strict

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
   "http://www.w3.org/TR/html4/strict.dtd">

Transitional

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
   "http://www.w3.org/TR/html4/loose.dtd">

Frameset

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"
   "http://www.w3.org/TR/html4/frameset.dtd">

XHTML 1.0

Strict

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

Transitional

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

Frameset

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">

XHTML 1.1

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 
   "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

See Also