View source for Document type declaration

A '''document type declaration''', or '''DOCTYPE''', is an instruction that associates a particular [[SGML]] or [[XML]] document (for example, a [[web page]]) with a [[document type definition]] (DTD) (for example, the formal definition of a particular version of [[HTML]]).

== Description ==

In the serialized form of the document, it manifests as a short string of markup that conforms to a particular [[syntax]].

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". 

Since web browsers are implemented with special-purpose HTML parsers, rather than general-purpose DTD-based parsers, they don't use DTDs and will never access them even if a URL is provided. 

== HTML5 ==

The text/html serialization of [[HTML5]], which is not SGML-based, uses the DOCTYPE only for mode selection. 

The DOCTYPE is retained in HTML5 as a "mostly useless, but required" header only to trigger "standards mode" in common browsers.

== Syntax ==

The general syntax for a document type declaration is:

<pre>
&lt;!DOCTYPE root-element PUBLIC "FPI" ["URI"] [ 
&lt;!-- internal subset declarations --&gt;
]&gt;
</pre>

or

<pre>
&lt;!DOCTYPE root-element SYSTEM "URI" [ 
&lt;!-- internal subset declarations --&gt;
]&gt;
</pre>

== See also ==

* [[Document]]
* [[Document type definition]]
* [[HTML]]
* [[HTML5]]
* [[Markup language]]
* [[SGML]]
* [[XHTML]]
* [[XML]]

== External Links ==

* [https://en.wikipedia.org/wiki/Document_type_declaration Document type declaration] @ Wikipedia

[[Category:Computing]]
[[Category:Markup languages]]
[[Category:Web design]]
[[Category:Web design and development]]