The 'application/xhtml+xml' Media Type

Network Working Group                                           M. Baker
Request for Comments: 3236                              Planetfred, Inc.
Category: Informational                                         P. Stark
                                          Ericsson Mobile Communications
                                                            January 2002


                 The 'application/xhtml+xml' Media Type

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document defines the 'application/xhtml+xml' MIME media type for
   XHTML based markup languages; it is not intended to obsolete any
   previous IETF documents, in particular RFC 2854 which registers
   'text/html'.

1. Introduction

   In 1998, the W3C HTML working group began work on reformulating HTML
   in terms of XML 1.0 [XML] and XML Namespaces [XMLNS].  The first part
   of that work concluded in January 2000 with the publication of the
   XHTML 1.0 Recommendation [XHTML1], the reformulation for HTML 4.01
   [HTML401].

   Work continues in the Modularization of XHTML Recommendation
   [XHTMLM12N], the decomposition of XHTML 1.0 into modules that can be
   used to compose new XHTML based languages, plus a framework for
   supporting this composition.

   This document only registers a new MIME media type,
   'application/xhtml+xml'.  It does not define anything more than is
   required to perform this registration.

   This document follows the convention set out in [XMLMIME] for the
   MIME subtype name; attaching the suffix "+xml" to denote that the
   entity being described conforms to the XML syntax as defined in XML
   1.0 [XML].

   This document was prepared by members of the W3C HTML working group
   based on the structure, and some of the content, of RFC 2854, the
   registration of 'text/html'.  Please send comments to www-
   html@w3.org, a public mailing list (requiring subscription) with
   archives at <http://lists.w3.org/Archives/Public/www-html/>.

2. Registration of MIME media type application/xhtml+xml

    MIME media type name:      application
    MIME subtype name:         xhtml+xml
    Required parameters:       none
    Optional parameters:

      charset
         This parameter has identical semantics to the charset parameter
         of the "application/xml" media type as specified in [XMLMIME].

      profile
         See Section 8 of this document.

   Encoding considerations:
      See Section 4 of this document.

   Security considerations:
      See Section 7 of this document.

   Interoperability considerations:
      XHTML 1.0 [XHTML10] specifies user agent conformance rules that
      dictate behaviour that must be followed when dealing with, among
      other things, unrecognized elements.

      With respect to XHTML Modularization [XHTMLMOD] and the existence
      of XHTML based languages (referred to as XHTML family members)
      that are not XHTML 1.0 conformant languages, it is possible that
      'application/xhtml+xml' may be used to describe some of these
      documents.  However, it should suffice for now for the purposes of
      interoperability that user agents accepting
      'application/xhtml+xml' content use the user agent conformance
      rules in [XHTML1].

      Although conformant 'application/xhtml+xml' interpreters can
      expect that content received is well-formed XML (as defined in
      [XML]), it cannot be guaranteed that the content is valid XHTML
      (as defined in [XHTML1]).  This is in large part due to the
      reasons in the preceding paragraph.

   Published specification:
      XHTML 1.0 is now defined by W3C Recommendation; the latest
      published version is [XHTML1].  It provides for the description of
      some types of conformant content as "text/html", but also doesn't
      disallow the use with other content types (effectively allowing
      for the possibility of this new type).

   Applications which use this media type:
      Some content authors have already begun hand and tool authoring on
      the Web with XHTML 1.0.  However that content is currently
      described as "text/html", allowing existing Web browsers to
      process it without reconfiguration for a new media type.

      There is no experimental, vendor specific, or personal tree
      predecessor to 'application/xhtml+xml'.  This new type is being
      registered in order to allow for the expected deployment of XHTML
      on the World Wide Web, as a first class XML application where
      authors can expect that user agents are conformant XML 1.0 [XML]
      processors.

   Additional information:

      Magic number:
         There is no single initial byte sequence that is always present
         for XHTML files.  However, Section 5 below gives some
         guidelines for recognizing XHTML files. See also section 3.1 in
         [XMLMIME].

      File extension:
         There are three known file extensions that are currently in use
         for XHTML 1.0; ".xht", ".xhtml", and ".html".

         It is not recommended that the ".xml" extension (defined in
         [XMLMIME]) be used, as web servers may be configured to
         distribute such content as type "text/xml" or
         "application/xml".  [XMLMIME] discusses the unreliability of
         this approach in section 3.  Of course, should the author
         desire this behaviour, then the ".xml" extension can be used.

      Macintosh File Type code: TEXT

   Person & email address to contact for further information:
      Mark Baker <mark.baker@canada.sun.com>

   Intended usage: COMMON

   Author/Change controller:
      The XHTML specifications are a work product of the World Wide Web
      Consortium's HTML Working Group.  The W3C has change control over
      these specifications.

3. Fragment identifiers

   URI references (Uniform Resource Identifiers, see [RFC2396] as
   updated by [RFC2732]) may contain additional reference information,
   identifying a certain portion of the resource. These URI references
   end with a number sign ("#") followed by an identifier for this
   portion (called the "fragment identifier"). Interpretation of
   fragment identifiers is dependent on the media type of the retrieval
   result.

   For documents labeled as 'text/html', [RFC2854] specified that the
   fragment identifier designates the correspondingly named element,
   these were identified by either a unique id attribute or a name
   attribute for some elements. For documents described with the
   application/xhtml+xml media type, fragment identifiers share the same
   syntax and semantics with other XML documents, see [XMLMIME], section
   5.

   At the time of writing, [XMLMIME] does not define syntax and
   semantics of fragment identifiers, but refers to "XML Pointer
   Language (XPointer)" for a future XML fragment identification
   mechanism. The current specification for XPointer is available at
   http://www.w3.org/TR/xptr. Until [XMLMIME] gets updated, fragment
   identifiers for XHTML documents designate the element with the
   corresponding ID attribute value (see [XML] section 3.3.1); any XHTML
   element with the "id" attribute.

4. Encoding considerations

   By virtue of XHTML content being XML, it has the same considerations
   when sent as 'application/xhtml+xml' as does XML.  See [XMLMIME],
   section 3.2.

5. Recognizing XHTML files

   All XHTML documents will have the string "<html" near the beginning
   of the document.  Some will also begin with an XML declaration which
   begins with "<?xml", though that alone does not indicate an XHTML
   document.  All conforming XHTML 1.0 documents will include an XML
   document type declaration with the root element type 'html'.

   XHTML Modularization provides a naming convention by which a public
   identifier for an external subset in the document type declaration of
   a conforming document will contain the string "//DTD XHTML".  And
   while some XHTML based languages require the doctype declaration to
   occur within documents of that type, such as XHTML 1.0, or XHTML
   Basic (http://www.w3.org/TR/xhtml-basic), it is not the case that all
   XHTML based languages will include it.

   All XHTML files should also include a declaration of the XHTML
   namespace.  This should appear shortly after the string "<html", and
   should read 'xmlns="http://www.w3.org/1999/xhtml"'.

6. Charset default rules

   By virtue of all XHTML content being XML, it has the same
   considerations when sent as 'application/xhtml+xml' as does XML.  See
   [XMLMIME], section 3.2.

7. Security Considerations

   The considerations for "text/html" as specified in [TEXTHTML] and and
   for 'application/xml' as specified in [XMLMIME], also hold for
   'application/xhtml+xml'.

   In addition, because of the extensibility features for XHTML as
   provided by XHTML Modularization, it is possible that
   'application/xhtml+xml' may describe content that has security
   implications beyond those described here.  However, if the user agent
   follows the user agent conformance rules in [XHTML1], this content
   will be ignored.  Only in the case where the user agent recognizes
   and processes the additional content, or where further processing of
   that content is dispatched to other processors, would security issues
   potentially arise.  And in that case, they would fall outside the
   domain of this registration document.

8. The "profile" optional parameter

   This parameter is meant to solve the short-term problem of using MIME
   media type based content negotiation (such as that done with the HTTP
   "Accept" header) to negotiate for a variety of XHTML based languages.
   It is intended to be used only during content negotiation.  It is not
   expected that it be used to deliver content, or that origin web
   servers have any knowledge of it (though they are welcome to).  It is
   primarily targeted for use on the network by proxies in the HTTP
   chain that manipulate data formats (such as transcoders).

   The parameter is intended to closely match the semantics of the
   "profile" attribute of the HEAD element as defined in [HTML401]
   (section 7.4.4.3), except it is applied to the document as a whole
   rather than just the META elements.  More specifically, the value of
   the profile attribute is a URI that can be used as a name to identify
   a language.  Though the URI need not be resolved in order to be
   useful as a name, it could be a namespace, schema, or a language
   specification.

   As an example, user agents supporting only XHTML Basic (see
   http://www.w3.org/TR/xhtml-basic) currently have no standard means to
   convey their inability to support the additional functionality in
   XHTML 1.0 [XHTML1] that is not found in XHTML Basic.  While XHTML
   Basic user agent conformance rules (which are identical to XHTML 1.0)
   provide some guidance to its user agent implementators for handling
   some additional content, the additional content in XHTML 1.0 that is
   not part of XHTML Basic is substantial, making those conformance
   rules insufficient for practical processing and rendering to the end
   user.  There is also the matter of the potentially substantial burden
   on the user agent in receiving and parsing this additional content.

   The functionality afforded by this parameter can also be achieved
   with at least two other more general content description frameworks;
   the "Content-features" MIME header described in RFC 2912, and UAPROF
   from the WAPforum (see http://www.wapforum.org/what/technical.htm).
   At this time, choosing one of these solutions would require excluding
   the other, as interoperability between the two has not been defined.
   For this reason, it is suggested that this parameter be used until
   such time as that issue has been addressed.

   An example use of this parameter as part of a HTTP GET transaction
   would be;

      Accept: application/xhtml+xml;
        profile="http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd"

9. Author's Address

   Mark A. Baker
   Planetfred, Inc.
   44 Byward Market, Suite 240
   Ottawa, Ontario, CANADA. K1N 7A2
   Phone: +1-613-789-1818
   EMail: mbaker@planetfred.com
   EMail: distobj@acm.org

   Peter Stark
   Ericsson Mobile Communications
   Phone: +464-619-3000
   EMail: Peter.Stark@ecs.ericsson.com

10.  References

   [HTML401]   Raggett, D., et al., "HTML 4.01 Specification", W3C
               Recommendation. Available at
               <http://www.w3.org/TR/html401> (or
               <http://www.w3.org/TR/1999/REC-html401-19991224>).

   [MIME]      Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part Two: Media Types", RFC 2046,
               November 1996.

   [URI]       Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
               Resource Identifiers (URI): Generic Syntax", RFC 2396,
               August 1998.

   [XHTML1]    "XHTML 1.0: The Extensible HyperText Markup Language: A
               Reformulation of HTML 4 in XML 1.0", W3C Recommendation.
               Available at <http://www.w3.org/TR/xhtml1>.

   [XML]       "Extensible Markup Language (XML) 1.0", W3C
               Recommendation.  Available at <http://www.w3.org/TR/REC-
               xml> (or <http://www.w3.org/TR/2000/REC-xml-20001006>).

   [TEXTHTML]  Connolly, D. and L. Masinter, "The 'text/html' Media
               Type", RFC 2854, June 2000.

   [XMLMIME]   Murata, M., St.Laurent, S. and D. Kohn, "XML Media
               Types", RFC 3023, January 2001.

   [XHTMLM12N] "Modularization of XHTML", W3C Recommendation. Available
               at: <http://www.w3.org/TR/xhtml-modularization>

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   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.