Document type declaration

<p>A document type declaration, or DOCTYPE, is an instruction that associates a particular <a href="/facts/XML/clNgUfWk">XML</a> or <a href="/facts/SGML/euIKXwHV">SGML</a> document (for example, a <a href="/facts/Web_page/5JK7xQLe">web page</a>) with a <a href="/facts/Document_type_definition/JsvY9PyQ">document type definition</a> (DTD) (for example, the formal definition of a particular version of <a href="/facts/HTML/sSc3fT78">HTML 2.0 - 4.0</a>). In the <a href="/facts/Serialize/T00auFvv">serialized</a> form of the document, it manifests as a short string of <a href="/facts/Markup_language/WwhlNVIu">markup</a> that conforms to a particular syntax.
</p><p>The <a href="/facts/HTML/sSc3fT78">HTML</a> <a href="/facts/Browser_engine/wLna3ate">layout engines</a> in modern <a href="/facts/Web_browser/2pfyymLs">web browsers</a> perform DOCTYPE "sniffing" or "switching", wherein the DOCTYPE in a document served as text/html determines a layout mode, such as "<a href="/facts/Quirks_mode/FUpVau8E">quirks mode</a>" or "standards mode". The text/html serialization of <a href="/facts/HTML5/HBAxEAr6">HTML5</a>, which is not SGML-based, uses the DOCTYPE only for mode selection.  Since web browsers are implemented with special-purpose HTML parsers, rather than general-purpose DTD-based parsers, they do not use DTDs and never access them even if a URL is provided.  The DOCTYPE is retained in HTML5 as a "mostly useless, but required" header only to trigger "standards mode" in common browsers.
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Document type declaration open-in-new

Document type declaration