Abstract
This document summarizes the best current practice for using various
Internet media types for serving various XHTML Family documents.
In summary, 'application/xhtml+xml'
SHOULD be used for XHTML Family documents, and
the use of 'text/html' SHOULD be limited to
HTML-compatible
XHTML 1.0 documents.
'application/xml' and 'text/xml' MAY also be used,
but whenever appropriate, 'application/xhtml+xml' SHOULD
be used rather than those generic
XML media types.
Status of This Document
This section describes the status of this document at the time of its
publication. Other documents may supersede this document. The latest
status of this document series is maintained at the W3C.
This document is a Note made available by the World Wide Web
Consortium (W3C) for your information. Publication of this Note
by W3C indicates no endorsement by W3C or the W3C Team, or any W3C Members.
This document represents the consensus of the
W3C HTML Working Group
(members only)
on the usage of Internet media types for various XHTML Family documents.
However, this document is not intended to be a normative specification.
Instead, it documents a set of recommendations to maximize
the interoperability of XHTML documents with regard to Internet media types.
This document does not address general issues on
media types and namespaces, which is outside the scope of
the HTML Working Group
charter and will be dealt by
the Technical Architecture Group
(TAG).
Comments on this document may be sent to
www-html-editor@w3.org
(archive).
Public discussion on this document may take place on the mailing list www-html@w3.org (archive).
This document has been produced as part of the W3C HTML Activity. The goals of
the HTML Working Group are discussed in the HTML Working Group charter.
A list of current W3C Recommendations
and other technical documents can be found at http://www.w3.org/TR.
XHTML 1.0 [XHTML1] reformulated
HTML 4 [HTML4] as an XML application,
and Modularization of XHTML [XHTMLM12N]
provided a means to define XHTML-based markup languages using
XHTML modules, collectively called as "XHTML Family".
However, due to historical reasons, a recommended way to serve such
XHTML Family documents, in particular with regard to Internet media
types, was quite unclear.
"5.1 Internet Media Type" of the first edition of
[XHTML1] included the following vague statement:
As of the publication of this recommendation, the general
recommended MIME labeling for XML-based applications
has yet to be resolved.
However, XHTML Documents which follow the guidelines set forth in
Appendix C, "HTML Compatibility Guidelines" may be
labeled with the Internet Media Type "text/html", as they
are compatible with most HTML browsers. This document
makes no recommendation about MIME labeling of other
XHTML documents.
Meanwhile, after the publication of [XHTML1],
an RFC for XML media
types was revised and published as RFC 3023
[RFC3023], and it introduced the '+xml'
suffix convention for XML-based media types.
The 'application/xhtml+xml' media type [RFC3236]
was registered following that convention.
Now that there are at least four possibilities on media type labeling for
XHTML Family documents - 'text/html', 'application/xhtml+xml', and
generic XML media types 'application/xml' and 'text/xml'.
This document summarizes the best current practice for using those various
Internet media types for various XHTML Family documents.
The key words "MUST", "MUST NOT",
"REQUIRED", "SHALL",
"SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED",
"MAY", and "OPTIONAL" in this document
are to be interpreted as described in RFC 2119
[RFC2119].
- XHTML
- The Extensible HyperText Markup Language.
XHTML is not the name of a single, monolithic markup language,
but the name of a family of document types which collectively form
this markup language. The namespace URI for XHTML is
http://www.w3.org/1999/xhtml.
- Note: Future version of XHTML might use
different namespace.
- XHTML Family document type
- A document type which belongs to a family of XHTML document types.
Such document types include [XHTML1],
and XHTML Host Language document types such as XHTML 1.1 [XHTML11] and XHTML Basic [XHTMLBasic].
Elements and attributes in those document types belong to the XHTML
namespace (except those from the XML namespace, such as
xml:lang), but an XHTML Family document type
MAY also include elements and attributes from other
namespaces, such as MathML
[MathML2].
- XHTML Host Language
document type
- A document type which conforms to the
"XHTML Host Language Document Type Conformance" as defined in
section 3.1 of [XHTMLM12N].
- XHTML Integration Set
document type
- A document type which conforms to the
"XHTML Integration Set Document Type Conformance" as defined in
section 3.2 of [XHTMLM12N].
This section summarizes which Internet media type SHOULD
be used for which XHTML Family document for which purpose.
The 'text/html' media type [RFC2854] is
primarily for HTML, not for XHTML. In general, this media type is
NOT suitable for XHTML.
However, as [RFC2854] says,
[XHTML1] defines a profile of use of XHTML which is
compatible with HTML 4.01 and which may also be labeled as text/html
.
[XHTML1], Appendix C
"HTML Compatibility Guidelines" summarizes
design guidelines
for authors who wish their XHTML documents to render on existing
HTML user agents
. The use of 'text/html' for XHTML
SHOULD be limited for the purpose of rendering on
existing HTML user agents, and SHOULD be limited to
[XHTML1] documents which follow
the HTML Compatibility Guidelines.
In particular, 'text/html' is NOT suitable for
XHTML Family document types that adds elements and attributes from
foreign namespaces, such as XHTML+MathML
[XHTML+MathML].
XHTML documents served as 'text/html' will not be processed as
XML [XML10], e.g. well-formedness errors
may not be detected by user agents. Also be aware that HTML rules will
be applied for DOM and
style sheets (see C.11 and C13 of [XHTML1]
respectively).
Authors should also be careful about character encoding issues.
A typical misunderstanding is that since an XHTML document is an XML
document, the character encoding of an XHTML document should be treated
as UTF-8 or UTF-16 in the absence of an explicit character encoding
information. This is NOT the case when an XHTML document
is served as 'text/html'. "6. Charset default rules" of
[RFC2854] notes as follows:
The use of an explicit charset parameter is strongly recommended.
While [MIME] specifies "The default character
set, which must be assumed in the absence of a charset parameter, is
US-ASCII." [HTTP]
Section 3.7.1, defines that "media subtypes of the 'text' type are
defined to have a default charset value of 'ISO-8859-1'". Section
19.3 of [HTTP] gives additional guidelines.
Using an explicit charset parameter will help avoid confusion.
Using an explicit charset parameter also takes into account that the
overwhelming majority of deployed browsers are set to use something
else than 'ISO-8859-1' as the default; the actual default is either a
corporate character encoding or character encodings widely deployed
in a certain national or regional community. For further
considerations, please also see Section 5.2 of
[HTML40].
"5.2.2 Specifying the character encoding" of the HTML 4 specification
[HTML4] also notes that
user agents
must not assume any default value for the "charset" parameter
.
Therefore, authors SHOULD NOT assume any default value
for an XHTML document served as 'text/html', and as mentioned in
[RFC2854], the use of an explicit charset
parameter is STRONGLY RECOMMENDED.
When it is difficult to specify an explicit charset parameter through
a higher-level protocol, authors SHOULD include
the XML declaration (e.g. <?xml version="1.0" encoding="EUC-JP"?>) and
a meta http-equiv statement (e.g. <meta http-equiv="Content-Type"
content="text/html; charset=EUC-JP" />).
See "C.9. Character Encoding" of [XHTML1]
for details.
The 'application/xhtml+xml' media type [RFC3236]
is the media type for XHTML Family document types, and
in particular it is suitable for XHTML Host Language document types.
XHTML Family document types suitable for this media type include
[XHTML1],
[XHTMLBasic],
[XHTML11] and
[XHTML+MathML].
An XHTML Host Language document type that adds elements and
attributes from foreign namespaces MAY identify
its profile with the 'profile' optional parameter or other means such as
the "Content-features" MIME header described in RFC 2912
[RFC2912].
Each namespace SHOULD be explicitly identified through
namespace declaration [XMLNS]. This document
does not preclude the registration of its own media type for specific
XHTML Host Language document type.
In general, this media type is NOT suitable for
XHTML Integration Set document types. This document does not define
which media type should be used for XHTML Integration Set document types.
'application/xhtml+xml' SHOULD be used for serving
XHTML documents to XHTML user agents. Authors who wish to support
both XHTML and HTML user agents MAY utilize content
negotiation by serving HTML documents as 'text/html' and XHTML documents
as 'application/xhtml+xml'. Also note that it is not necessary
for XHTML documents served as 'application/xhtml+xml' to follow
the HTML Compatibility Guidelines.
When serving an XHTML document with this media type, authors
SHOULD include the XML stylesheet processing instruction
[XMLstyle] to associate style sheets.
As for character encoding issues, as mentioned in "6. Charset default
rules" of [RFC3236], 'application/xhtml+xml'
has the same considerations as 'application/xml'. See
section 3.3 for details.
The 'application/xml' media type [RFC3023]
is a generic media type for XML documents, and the definition of
'application/xml' does not preclude serving XHTML documents as that
media type. Any XHTML Family document MAY be served
as 'application/xml'.
However, authors should be aware that such a document may not
always be processed as XHTML (e.g. hyperlinks may not be recognized),
depending on user agents. Generic XML processors might recognize it
as just an XML document which includes elements and attributes from
the XHTML namespace (and others), and may not have a priori knowledge
what to do with such a document beyond they can do for generic XML
documents.
Authors SHOULD explicitly identify the XHTML namespace
through the namespace declaration when they serve an XHTML Family document as
'application/xml' to facilitate the chance for reliable processing. The XML
stylesheet PI
SHOULD be used to associate style sheets.
Whenever appropriate, 'application/xhtml+xml' SHOULD
be used rather than 'application/xml'.
As for character encoding issues, "3.2 Application/xml Registration" of
[RFC3023] says that
the use of
the charset parameter is STRONGLY RECOMMENDED
, and
also specifies a rule that
[i]f an application/xml
entity is received where the charset parameter is omitted, no information
is being provided about the charset by the MIME Content-Type header
.
This means that conforming XML processors MUST follow
the requirements described in section 4.3.3 of
[XML10].
Therefore, while it is STRONGLY RECOMMENDED to
specify an explicit charset parameter through a higher-level protocol,
authors SHOULD include the XML declaration
(e.g. <?xml version="1.0" encoding="EUC-JP"?>).
Note that a meta http-equiv statement will not be recognized by XML
processors, and authors SHOULD NOT include such
a statement in an XHTML document served as 'application/xml' (and
'application/xhtml+xml' as well for that matter).
The 'text/xml' media type [RFC3023] is
an another generic media type for XML documents, and the definition of
'text/xml' does not preclude serving XHTML documents as that media type,
either. Any XHTML Family document MAY be served as 'text/xml'.
The considerations for 'application/xml' also apply to 'text/xml'.
Whenever appropriate, 'application/xhtml+xml' SHOULD
be used rather than 'text/xml'.
Authors should also be aware of the difference between
'application/xml' (and for that matter 'application/xhtml+xml' as well)
and 'text/xml' with regard to the treatment of character encoding.
According to "3.1 Text/xml Registration" of
[RFC3023],
if a text/xml entity
is received with the charset parameter omitted, MIME processors and XML
processors MUST use the default charset value of
"us-ascii"[ASCII]
.
This default value is authoritative over the encoding information
specified in the XML declaration, or the XML default encodings of
UTF-8 and UTF-16 when no encoding declaration is supplied, so omitting
the charset parameter of a 'text/xml' entity might cause an unexpected
result.
As mentioned in [RFC3023], the use of the charset
parameter is STRONGLY RECOMMENDED.
The following table summarizes recommendation to content authors
for labeling XHTML documents. HTML 4 is also listed for comparison
purpose.
Media types summary for serving XHTML documents
| Media type |
HTML 4 |
XHTML 1.0 (HTML compatible) |
XHTML 1.0 (other) |
XHTML Basic / 1.1 |
XHTML+MathML |
| text/html |
SHOULD |
MAY |
SHOULD NOT |
SHOULD NOT |
SHOULD NOT |
| application/xhtml+xml |
MUST NOT |
SHOULD |
SHOULD |
SHOULD |
SHOULD |
| application/xml |
MUST NOT |
MAY |
MAY |
MAY |
MAY |
| text/xml |
MUST NOT |
MAY |
MAY |
MAY |
MAY |
- [ASCII]
- "Information Systems -- Coded Character Sets -- 7-Bit American
National Standard Code for Information Interchange (7-Bit ASCII)",
ANSI
X3.4-1986, 1986.
- [HTML4]
"HTML 4.01
Specification", W3C Recommendation, D. Raggett, A. Le Hors,
I. Jacobs, eds., 24 December 1999.
Available at: http://www.w3.org/TR/1999/REC-html401-19991224
The latest version of HTML 4.01
is available at: http://www.w3.org/TR/html401
The latest version of HTML 4
is available at: http://www.w3.org/TR/html4
- [HTML40]
- "HTML 4.0
Specification", W3C Recommendation, D. Raggett, A. Le Hors,
I. Jacobs, eds., 18 December 1997, revised
on 24 April 1998.
Available at http://www.w3.org/TR/1998/REC-html40-19980424
- [HTTP]
- "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, J. Gettys, R. Fielding,
J. Mogul, H. Frystyk, L. Masinter, P. Leach and T. Berners-Lee, June 1999.
Available at: http://www.rfc-editor.org/rfc/rfc2616.txt
- [MathML2]
"Mathematical
Markup Language (MathML) Version 2.0", W3C Recommendation,
D. Carlisle, P. Ion, R. Miner, N. Poppelier, eds.,
21 February 2001.
Available at: http://www.w3.org/TR/2001/REC-MathML2-20010221
The latest version is
available at: http://www.w3.org/TR/MathML2
- [MIME]
- "Multipurpose
Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046,
N. Freed, N. Borenstein, November 1996.
Available at: http://www.rfc-editor.org/rfc/rfc2046.txt
- [RFC2119]
- "Key words for
use in RFCs to Indicate Requirement Levels", RFC 2119, S. Bradner,
March 1997.
Available at: http://www.rfc-editor.org/rfc/rfc2119.txt
- [RFC2854]
- "The 'text/html'
Media Type", RFC 2854, D. Connolly, L. Masinter, June 2000.
Available at: http://www.rfc-editor.org/rfc/rfc2854.txt
- [RFC2912]
- "Indicating
Media Features for MIME Content", RFC 2912, G. Klyne, September 2000.
Available at: http://www.rfc-editor.org/rfc/rfc2912.txt
- [RFC3023]
- "XML Media Types",
RFC3023, M. Murata, S. St.Laurent, D. Kohn, January 2001.
Available at: http://www.rfc-editor.org/rfc/rfc3023.txt
- [RFC3236]
- "The
'application/xhtml+xml' Media Type", RFC 3236, M. Baker, P. Stark,
January 2002. Available at: http://www.rfc-editor.org/rfc/rfc3236.txt
- [XHTML1]
"XHTML™ 1.0 The Extensible HyperText Markup Language (Second Edition):
A Reformulation of HTML 4 in XML 1.0", W3C Recommendation,
S. Pemberton et al., August 2002.
Available at: http://www.w3.org/TR/2002/REC-xhtml1-20020801
The first edition
is available at: http://www.w3.org/TR/2000/REC-xhtml1-20000126
The latest version is
available at: http://www.w3.org/TR/xhtml1
- [XHTML11]
"XHTML™
1.1 - Module-based XHTML", W3C Recommendation,
M. Altheim, S. McCarron, eds., 31 May 2001.
Available at: http://www.w3.org/TR/2001/REC-xhtml11-20010531
The latest version
is available at: http://www.w3.org/TR/xhtml11
- [XHTMLBasic]
"XHTML™ Basic", W3C Recemmendation,
M. Baker, M. Ishikawa, S. Matsui, P. Stark, T. Wugofski, T. Yamakami,
eds., 19 December 2000.
Available at: http://www.w3.org/TR/2000/REC-xhtml-basic-20001219
The latest version is
available at: http://www.w3.org/TR/xhtml-basic
- [XHTMLM12N]
"Modularization of XHTML™", W3C Recommendation,
M. Altheim, F. Boumphrey, S. Dooley, S. McCarron, S. Schnitzenbaumer,
T. Wugofski, eds., 10 April 2001.
Available at: http://www.w3.org/TR/2001/REC-xhtml-modularization-20010410
The latest version
is at: http://www.w3.org/TR/xhtml-modularization
- [XHTML+MathML]
- "XHTML
plus Math 1.1 DTD",
"A.2 MathML as a DTD Module", Mathematical Markup Language (MathML)
Version 2.0. Available at:
http://www.w3.org/TR/MathML2/dtd/xhtml-math11-f.dtd
- [XML10]
"Extensible
Markup Language (XML) 1.0 Specification (Second Edition)", T. Bray,
J. Paoli, C. M. Sperberg-McQueen, E. Maler, eds., 6 October 2000.
Available at: http://www.w3.org/TR/2000/REC-xml-20001006
The latest version is
available at: http://www.w3.org/TR/REC-xml
- [XMLNS]
"Namespaces
in XML", T. Bray, D. Hollander, A. Layman, eds.,
14 January 1999.
Available at: http://www.w3.org/TR/1999/REC-xml-names-19990114
The latest version is
available at: http://www.w3.org/TR/REC-xml-names
- [XMLstyle]
"Associating Style Sheets with XML documents Version 1.0",
W3C Recommendation, J. Clark, ed.,
29 June 1999. Available at:
http://www.w3.org/1999/06/REC-xml-stylesheet-19990629
The latest version is
available at: http://www.w3.org/TR/xml-stylesheet
In 3.5. Summary, changed 'text/html' for
HTML 4 as SHOULD rather than MAY.
Updated reference to XHTML 1.0 to refer
to the Second Edition.