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A Document Type Declaration, or DOCTYPE, is an instruction that associates a particular SGML or XML document (for example, a webpage) with a Document Type Definition (DTD) (for example, the formal definition of a particular version of HTML). In the serialized form of the document, it manifests as a short string of markup that conforms to a particular syntax.

Despite its name, a Document Type Declaration is not suitable for deducing the type of the document, although apparently it was originally supposed to be.

The HTML layout engines in modern web browsers perform DOCTYPE "sniffing" or "switching", wherein the DOCTYPE in a document served as text/html determines a layout mode, such as "quirks mode" or "standards mode".

DTDs

Syntax

The general syntax for a Document Type Declaration is:

<!DOCTYPE root-element [SYSTEM OR PUBLIC FPI] "uri" [ 
<!-- internal declarations -->
]>

In XML, the root element of the document is the first element in the document (In XHTML, the root element is html - being the first element opened and last closed). The keywords SYSTEM and PUBLIC suggest what kind of DTD it is (one that is on a private system or one that is open to the public). If the "SYSTEM" keyword is chosen, the URI of the DTD may follow; if the PUBLIC keyword is chosen then the declaration must have a FPI (Formal Public Identifier) following it and then the URI of the DTD (the FPI for XHTML 1.1 is "-//W3C//DTD XHTML 1.1//EN").

The last part is called an internal subset which can be used to add/edit entities or add/edit elements. This is optional in XML.

Document Type Declarations are slightly different in SGML, where you may associate the public identifier with the system identifier, as it is in HTML.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">

Example

The first line of many World Wide Web pages reads as follows:

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

This Document Type Declaration for XHTML includes by reference a DTD, whose public identifier is -//W3C//DTD XHTML 1.0 Transitional//EN and whose system identifier is http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd. An entity resolver may use either identifier for locating the referenced external entity. The root element is declared to be html.

HTML 4.01 DTDs

HTML 4.01 Strict does not allow presentational markup with the argument that Cascading Style Sheets should be used for that instead. This is how the strict doctype looks:

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

Transitional DTD allows some older elements and attributes that have been deprecated:

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

Also if frames are used, to get valid results from the SGML validator, the frameset doctype is needed, like this:

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

XHTML 1.0 DTDs

XHTML's DTDs are also Strict, Transitional and Frameset.

XHTML Strict DTD is the most strict DTD available: no deprecated tags are supported and the code must be written correctly.

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

XHTML Transitional DTD is like the XHTML Strict DTD, but deprecated tags are allowed. This is the most popular current DTD.^{[citation needed]}

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

XHTML Frameset DTD is the only XHTML DTD that supports Frameset. The DTD is below.

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

XHTML 1.1 DTD

This is the latest DTD that has the stringency of XHTML 1.0 Strict, and it is based upon the module framework and modules defined in Modularization of XHTML.

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

XHTML Basic DTDs

XHTML Basic 1.0

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
 "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">

XHTML Basic 1.1

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.1//EN"
 "http://www.w3.org/TR/xhtml-basic/xhtml-basic11.dtd">

XHTML Mobile Profile DTDs

XHTML Mobile Profile 1.0

<!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.0//EN" 
"http://www.wapforum.org/DTD/xhtml-mobile10.dtd">

XHTML Mobile Profile 1.1

<!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.1//EN" 
"http://www.openmobilealliance.org/tech/DTD/xhtml-mobile11.dtd">

XHTML Mobile Profile 1.2

<!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.2//EN" 
"http://www.openmobilealliance.org/tech/DTD/xhtml-mobile11.dtd">

External links

Recommended DTDs to use in your Web document - an informative (not normative) W3C Quality Assurance publication
Doctype switch: summary table - an overview table of the effect of different DOCTYPEs on selecting quirks mode or standards compliance mode in HTML renderers
DOCTYPE grid - another overview table
FreeBSD Documentation Project Primer section on DOCTYPE - has some info about syntax and formal public identifiers
Quirks mode and strict mode
Box model tweaking
W3C HTML Validator

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 A '''Document Type Declaration''', or '''DOCTYPE''', is an instruction that associates a particular [[SGML]] or [[XML]] document (for example, a [[webpage]]) with a [[Document Type Definition]] (DTD) (for example, the formal definition of a particular version of [[HTML]]). In the serialized form of the document, it manifests as a short string of [[markup]] that conforms to a particular syntax.
-Despite its name, a Document Type Declaration [http://groups.google.fi/groups?selm=34E9CBC9.401B6BB0%40isogen.com&output=gplain is not suitable for deducing the type of the document], although apparently it was originally supposed to be.
+Despite its name, a Document Type Declaration [http://groups.google.com/groups?selm=34E9CBC9.401B6BB0%40isogen.com&output=gplain is not suitable for deducing the type of the document], although apparently it was originally supposed to be.
 The [[HTML]] [[layout engine]]s in modern [[web browser]]s perform DOCTYPE "sniffing" or "switching", wherein the DOCTYPE in a document served as <code>text/html</code> determines a layout mode, such as "[[quirks mode]]" or "standards mode".