This document summarizes the current best practice for using various Internet media types when serving XHTML Family documents to relatively modern user agents - even those that do not yet support XHTML natively. 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 Family documents intended for delivery to user agents that do not explcitly state in their HTTP Accept header that they accept 'application/xhtml+xml'. The media types 'application/xml' and 'text/xml' MAY also be used, but whenever appropriate, 'application/xhtml+xml' or 'text/html' SHOULD be used rather than those generic XML media types.
Note that, because of the lack of explicit support for XHTML (and XML in general) in some user agents, only very careful construction of documents can ensure their portability (see Appendix A). If you do not require the advanced features of XHTML Family markup languages (e.g., XML DOM, XML Validation, extensibility via XHTML Modularization, semantic markup via XHTML+RDFa, Assistive Technology access via the XHTML Role and XHTML Access modules, etc.), you may want to consider using HTML 4.01 [HTML] in order to reduce the risk that content will not be portable to HTML user agents. Even in that case authors can help ensure their portability AND ease their eventual migration to the XHTML Family by ensuring their documents are valid [VALIDATOR] and by following the relevant guidelines in Appendix A.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
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 has been produced by the W3C XHTML 2 Working Group as part of the HTML Activity. The goals of the XHTML 2 Working Group are discussed in the XHTML 2 Working Group charter. The document represents working group consensus 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.
Comments on this document may be sent to email@example.com (archive). Public discussion on this document may take place on the mailing list firstname.lastname@example.org (archive).
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 the "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 somewhat unclear.
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 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 current best practice for using those various Internet media types for 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].
xml:lang), but an XHTML Family document type MAY also include elements and attributes from other namespaces, such as MathML [MathML2].
This section summarizes which Internet media type SHOULD be used for which XHTML Family document for which purpose.
A combination of these rules, in conjunction with a careful examination of the HTTP Accept header, can be useful in determining which media type to use when a document adheres to the guidelines in Appendix A. Specifically:
application/xhtml+xmldeliver the document using that media type.
text/html, deliver the document using that media type.
In other words, requestors that advertise they support XHTML family documents will receive the document in the XHTML media type, and all other requestors will receive the document using the HTML media type.
When a document does NOT adhere to the guidelines, it SHOULD NOT
be delivered as media type
text/html. If such documents need to be
delivered to requestors who do not explicitly support
the XHTML family, those documents should be transformed into
valid HTML and then delivered as such.
Note: It is possible that in the future XHTML Modularization
will define rules for indicating which specific XHTML family members are
supported by a requestor (e.g., via the
of the media type in the Accept header). Such rules, when used in conjunction
with the "quality" parameter of the media type could help a server determine
which of several versions of a document to deliver.
The 'text/html' media type [RFC2854] is primarily for HTML, not for XHTML. In general, this media type is NOT suitable for XHTML except when the XHTML is carefully constructed (see Appendix A. In particular, 'text/html' is NOT suitable for XHTML Family document types that add 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 guidelines 11 and 13).
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
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 (e.g., HTTP), 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 guideline 9 for details.
The 'application/xhtml+xml' media type [RFC3236] is the primary media type for XHTML Family document types, and in particular it is suitable for all 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 (agents that explicitly indicate their support for this media type). Authors who wish to support both XHTML and HTML user agents MAY utilize content negotiation by serving carefully constructed XHTML documents both as 'text/html' and as 'application/xhtml+xml'. Alternately, authors may serve HTML versions of such documents as 'text/html' and XHTML versions as 'application/xhtml+xml'. Also note that it is not necessary for XHTML documents served as 'application/xhtml+xml' to follow the HTML4 Compatibility Guidelines.
When serving an XHTML document with this media type, authors MAY include the XML stylesheet processing instruction [XMLstyle] to associate style sheets. This is not generally necessary when documents are to be processed by XHTML-aware user agents, but generic XML document processors may handle such processing instructions.
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
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 while authors MAY include such a statement a statement in an XHTML document served as 'application/xml' it will not effect processing of the document since the higher level protocol and the XML PI both take precedence.
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
if a text/xml entity
is received with the charset parameter omitted, MIME processors and XML
processors MUST use the default charset value of
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
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.
|Media type||HTML 4||XHTML Family (HTML 4 compatible)||XHTML Family (other)||XHTML Family + Extensions|
|text/html||SHOULD||MAY||SHOULD NOT*||SHOULD NOT|
* However, see transformation.
This appendix summarizes design guidelines for authors who wish their XHTML documents to render on both XHTML-aware and modern HTML user agents. The purpose of providing these guidelines is to supply a simple collection that, if followed, will give reasonable, predictable results in modern user agents. Document authors should treat these as best practices that were considered correct at the time this document was published. Like all of this document, this Appendix is informative. It contains no absolute requirements, and should NEVER be used as the basis for creating conformance nor validation rules of any sort. Period.
For an example document that reflect the use of the guidelines from this section, see Appendix B.
DO NOT include XML processing instructions NOR the XML declaration.
Rationale: Some HTML user agents render XML processing instructions. Also, some user agents interpret the XML declaration to mean that the document is unrecognized XML rather than HTML. Such user agents may not render the document as expected. For compatibility with these types of HTML browsers, you should avoid using processing instructions and XML declarations.
Consequence: Remember, however, that when the XML declaration is not included in a document, AND the character encoding is not specified by a higher level protocol such as HTTP, the document can only use the default character encodings UTF-8 or UTF-16. See, however, guideline 9 below.
If an element has an EMPTY content model DO use the minimized tag syntax
permitted by XML (e.g.,
DO NOT use the alternative syntax (e.g.,
<br></br>) allowed by XML, since this may be unsupported by
HTML user agents.
Also, DO include a space before the trailing
Empty elements in the XHTML family include:
Rationale: HTML user agents ignore the
the end of a tag, but without it they may incorrectly parse the tag or its attributes. HTML user agents also may not recognize the alternate
syntax permitted by XML.
If an element permits content (e.g., the
but an instance of that element has no content (e.g., an empty
paragraph), DO NOT use the
"minimized" tag syntax (e.g.,
Rationale: HTML user agents may give uncertain results when using the the minimized syntax permitted by XML when an element has no content.
DO use external style sheets if your style sheet uses
DO NOT use an internal stylesheet
if the style rules contain any of the above characters.
external scripts if your script uses
DO NOT embed a script in a document
if it contains any of these characters.
Rationale: XML parsers are permitted to silently remove the contents of comments. Therefore, the historical practice of "hiding" scripts and style sheets within "comments" to make the documents backward compatible may not work as expected in XML-based user agents.
@@@@Put a real example in here that works, and one that does not work@@@@
DO ensure that attribute values are on a single line and only use single whitespace characters. DO NOT use line breaks and multiple consecutive white space characters within attribute values.
Rationale: These are handled inconsistently by user agents.
DO use both
attributes when specifying the language of an element in markup languages that support the use of both.
DO NOT use the only
lang attribute, even in languages that
include it such as XHTML 1.0.
HTML 4 documents use the
to identify the language
of an element. XML documents use the
CSS has a "lang" pseudo selector that automatically uses the appropriate
attribute depending on the document type. Therefore, specifying both
attributes ensures that single CSS selectors will work in both modes.
DO use the
id attribute to identify elements.
DO ensure that the values used for the
id attribute are limited to the pattern
DO NOT use the
name attribute to identify elements, even in languages that
permit the use of
name such as XHTML 1.0.
Rationale: In HTML 3.2 and earlier the
name attribute on some elements could
be used to define an anchor, but HTML 4 introduced the
id attribute. In an XML dialect, only attributes
ID are permitted to be used
as anchors, and the
id attribute is defined to be
ID. Relying upon the
as an anchor will work well in modern HTML and
XHTML-aware user agents.
DO encode your document in UTF-8 or UTF-16. When delivering the document from a server, DO set the character encoding for a document via the charset parameter of the HTTP Content-Type header. When not delivering the document from a server, DO set the encoding via a "meta http-equiv" statement in the document (e.g., <meta http-equiv="Content-Type" content="text/html; charset=EUC-JP" />). However, note that doing so will explicitly bind the document to an a single content type.
Rationale: Since these guidelines already recommend that documents NOT contain the XML declaration, setting the encoding via the HTTP header is the only reliable mechanism compatible with HTML and XML user agents. When that mechanism is not available, the only portable fallback is the "meta http-equiv" statement.
DO use the full form for boolean attributes, as required by
Such attributes include:
Rationale: The compact form of these attributes is not well formed XML, and therefore invalid.
DO rely upon the HTML 4 DOM as defined in The Document Object Model level 1 Recommendation [DOM] for scripting. This means, in particular, that the names of elements and attributes will be returned (from functions that return such things) in upper case.
Rationale: Using the HTML DOM will result in maximum portability of scripts, since the HTML DOM is supported in both HTML and XHTML documents in modern user agents.
DO ensure that when content or attribute values contain
the reserved character
it is used in its escaped form
Rationale: If ampersands are not encoded, the characters after them up to the next semi-colon can be interpreted as the name of a entity by the user agent.
DO use lower case element and attribute names in style sheets.
DO create rules that include inferred elements (e.g., the
element in a table).
Rationale: These simple rules will help increase the portability of CSS rules regardless of the media type the document is processed as.
DO NOT use xml stylesheet declarations to identify style sheets.
DO use the
link elements to
Rationale: Since XML processing instructions may be
rendered by some HTML user agents, using the standard XML stylesheet
declaration mechanism may not work well. However, since XHTML user agents
are required to process
and interpret stylesheets referenced from those elements, documents
constructed to use them will work as expected.
DO NOT use the formfeed character (U+000C).
Rationale: This character is recognized as white space in HTML 4, but is NOT considered white space in XML.
' to specify an escaped apostrophe.
DO NOT use
Rationale: The entity
' is not defined
in HTML 4.
The following is an example document that adopts the conventions described in Appendix A to ensure its portability among XHTML and HTML user agents.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"> <head> <title>sample</title> <link href="style/style.css" rel="stylesheet" type="text/css" /> </head> <body> <div id="main"> <h1>heading</h1> <img src="http://www.w3.org/Icons/w3c_main" alt="W3C logo" /> <!-- defined as an "EMPTY" element, do not use <img></img> or <img/> --> <p>Some material & some <!-- use escaped ampersand, & --> <br /> <!-- defined as an "EMPTY" element, do not use <br></br> or <br/> --> that should be split.</p> <p></p> <!-- NOT defined as an "EMPTY" element, just no content, so do not use <p/> nor <p /> --> <input type="reset" disabled="disabled" /> <!-- defined as an "EMPTY" element, do not use <hr></hr> nor <hr/> --> <hr /> <!-- defined as an "EMPTY" element, do not use <hr></hr> nor <hr/> --> </div> </body> </html>
"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
"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
The W3C Markup Validation Service available at http://validator.w3.org.
"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
"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
"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
"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
"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
"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
"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.