What is Document Type Declaration or DOCTYPE?

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". The text/html serialization of HTML 5, which is not SGML-based, uses DOCTYPE only for mode selection.