The MIME type application/xhtml+xml is used to explicitly denote the encoding required for the file type XHTML. The category of this media type is "application".

The XHTML file type is used for Extensible Hyper Text Markup Language. It is a form of HTML language that has the same depth of expression as HTML but also conforms to the XML syntax, which is far stricter. XHTML is thus an application of XML, which is a far stricter subset of SGML, on which HTML is based. True XHTML files, due to their strict formation will allow for automatic processing through standard XML tools whereas normal HTML files would require a far more lenient, complex and generally custom parser. The first version of XHTML (1.0) achieved a W3C recommendation on January 26, 2000.

The XHTML version 1.0 was not very different from HTML and the main changes that were made were made in order to conform to XML. The important changes were the necessity of the document to be "well-formed" and with explicitly closed tags, just like they in XML. It also prohibits attribute minimization and requires all attributes to be explicitly assigned.