Document Type Declaration - DTDs - HTML5 DTD-less DOCTYPE

HTML5 DTD-less DOCTYPE

HTML5 uses a DOCTYPE declaration which is very short, due to its lack of references to a Document Type Definition in the form of a URL or FPI. All it contains is the tag name of the root element of the document, HTML. In the words of the specification draft itself:

In other words, , case-insensitively.

With the exception of the lack of a URI or the FPI string (the FPI string is treated case sensitively by validators), this format (a case-insensitive match of the string !DOCTYPE HTML) is the same as found in the syntax of the SGML based HTML 4.01 DOCTYPE. Both in HTML4 and in HTML5, the formal syntax is defined in upper case letters, even if both lower case and mixes of lower case upper case are also treated as valid.

In XHTML5 the DOCTYPE must be a case-sensitive match of the string "". This is because in XHTML syntax all HTML PUBLIC are required to be in lower case, including the root element referenced inside the HTML5 DOCTYPE. As well, XHTML only accepts the upper case inside the DOCTYPE string. These rules are not defined by the HTML5 specification itself but by XML and the syntax rules for XHTML DTDs. For the XHTML5 syntax, then Document Type Definitions are permitted as well.

The DOCTYPE is optional in XHTML5 and may simply be omitted. However, if the markup is to be processed as both XML and HTML, a DOCTYPE should be used. Given that the HTML5 specification forbids XML-serialized HTML5 (XHTML5) from being served with any MIME type other than application/xhtml+xml, this is unlikely to be a situation encountered in the real-world. Unlike with the previous versions of XHTML, it is impossible to serve an XHTML5 (that is, HTML5 serialized as XML) document as text/html in any conceivable situation; any situation involving XHTML5 will be served as application/xhtml+xml and parsed as XML in a standards-compliant system.