Please refer to the errata for this document, which may include some normative corrections. See also translations.
This document is also available in these non-normative formats: Multi-part XHTML file, PostScript version, PDF version, ZIP archive, and Gzip'd TAR archive.
Copyright ©2002 W3C® (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
This specification defines the Second Edition of XHTML 1.0, a reformulation of HTML 4 as an XML 1.0 application, and three DTDs corresponding to the ones defined by HTML 4. The semantics of the elements and their attributes are defined in the W3C Recommendation for HTML 4. These semantics provide the foundation for future extensibility of XHTML. Compatibility with existing HTML user agents is possible by following a small set of guidelines.
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 the second edition of the XHTML 1.0 specification incorporating the errata changes as of 1 August 2002. Changes between this version and the previous Recommendation are illustrated in a diff-marked version.
This second edition is not a new version of XHTML 1.0 (first published 26 January 2000). The changes in this document reflect corrections applied as a result of comments submitted by the community and as a result of ongoing work within the HTML Working Group. There are no substantive changes in this document - only the integration of various errata.
The list of known errors in this specification is available at http://www.w3.org/2002/08/REC-xhtml1-20020801-errata.
Please report errors in this document to www-html-editor@w3.org (archive). Public discussion on HTML features takes 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 (members only) are discussed in the HTML Working Group charter.
At the time of publication, the working group believed there were zero patent disclosures relevant to this specification. A current list of patent disclosures relevant to this specification may be found on the Working Group's patent disclosure page.
A list of current W3C Recommendations and other technical documents can be found at
http://www.w3.org/TR.
lang
and xml:lang
AttributesThis section is informative.
XHTML is a family of current and future document types and modules that reproduce, subset, and extend HTML 4 [HTML4]. XHTML family document types are XML based, and ultimately are designed to work in conjunction with XML-based user agents. The details of this family and its evolution are discussed in more detail in [XHTMLMOD].
XHTML 1.0 (this specification) is the first document type in the XHTML family. It is a reformulation of the three HTML 4 document types as applications of XML 1.0 [XML]. It is intended to be used as a language for content that is both XML-conforming and, if some simple guidelines are followed, operates in HTML 4 conforming user agents. Developers who migrate their content to XHTML 1.0 will realize the following benefits:
The XHTML family is the next step in the evolution of the Internet. By migrating to XHTML today, content developers can enter the XML world with all of its attendant benefits, while still remaining confident in their content's backward and future compatibility.
HTML 4 [HTML4] is an SGML (Standard Generalized Markup Language) application conforming to International Standard ISO 8879, and is widely regarded as the standard publishing language of the World Wide Web.
SGML is a language for describing markup languages, particularly those used in electronic document exchange, document management, and document publishing. HTML is an example of a language defined in SGML.
SGML has been around since the middle 1980's and has remained quite stable. Much of this stability stems from the fact that the language is both feature-rich and flexible. This flexibility, however, comes at a price, and that price is a level of complexity that has inhibited its adoption in a diversity of environments, including the World Wide Web.
HTML, as originally conceived, was to be a language for the exchange of scientific and other technical documents, suitable for use by non-document specialists. HTML addressed the problem of SGML complexity by specifying a small set of structural and semantic tags suitable for authoring relatively simple documents. In addition to simplifying the document structure, HTML added support for hypertext. Multimedia capabilities were added later.
In a remarkably short space of time, HTML became wildly popular and rapidly outgrew its original purpose. Since HTML's inception, there has been rapid invention of new elements for use within HTML (as a standard) and for adapting HTML to vertical, highly specialized, markets. This plethora of new elements has led to interoperability problems for documents across different platforms.
XML™ is the shorthand name for Extensible Markup Language [XML].
XML was conceived as a means of regaining the power and flexibility of SGML without most of its complexity. Although a restricted form of SGML, XML nonetheless preserves most of SGML's power and richness, and yet still retains all of SGML's commonly used features.
While retaining these beneficial features, XML removes many of the more complex features of SGML that make the authoring and design of suitable software both difficult and costly.
The benefits of migrating to XHTML 1.0 are described above. Some of the benefits of migrating to XHTML in general are:
This section is normative.
The following terms are used in this specification. These terms extend the definitions in [RFC2119] in ways based upon similar definitions in ISO/IEC 9945-1:1990 [POSIX.1]:
This section is normative.
This version of XHTML provides a definition of strictly conforming XHTML 1.0 documents, which are restricted to elements and attributes from the XML and XHTML 1.0 namespaces. See Section 3.1.2 for information on using XHTML with other namespaces, for instance, to include metadata expressed in RDF within XHTML documents.
A Strictly Conforming XHTML Document is an XML document that requires only the facilities described as mandatory in this specification. Such a document must meet all of the following criteria:
It must conform to the constraints expressed in one of the three DTDs found in DTDs and in Appendix B.
The root element of the document must be html
.
The root element of the document must contain an xmlns
declaration for the XHTML namespace [XMLNS]. The namespace for XHTML is
defined to be http://www.w3.org/1999/xhtml
. An example root element might look like:
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
There must be a DOCTYPE declaration in the document prior to the root element. The public identifier included in the DOCTYPE declaration must reference one of the three DTDs found in DTDs using the respective Formal Public Identifier. The system identifier may be changed to reflect local system conventions.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
The DTD subset must not be used to override any parameter entities in the DTD.
An XML declaration is not required in all XML documents; however XHTML document authors are strongly encouraged to use XML declarations in all their documents. Such a declaration is required when the character encoding of the document is other than the default UTF-8 or UTF-16 and no encoding was determined by a higher-level protocol. Here is an example of an XHTML document. In this example, the XML declaration is included.
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <title>Virtual Library</title> </head> <body> <p>Moved to <a href="http://example.org/">example.org</a>.</p> </body> </html>
The XHTML namespace may be used with other XML namespaces as per [XMLNS], although such documents are not strictly conforming XHTML 1.0 documents as defined above. Work by W3C is addressing ways to specify conformance for documents involving multiple namespaces. For an example, see [XHTML+MathML].
The following example shows the way in which XHTML 1.0 could be used in conjunction with the MathML Recommendation:
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <title>A Math Example</title> </head> <body> <p>The following is MathML markup:</p> <math xmlns="http://www.w3.org/1998/Math/MathML"> <apply> <log/> <logbase> <cn> 3 </cn> </logbase> <ci> x </ci> </apply> </math> </body> </html>
The following example shows the way in which XHTML 1.0 markup could be incorporated into another XML namespace:
<?xml version="1.0" encoding="UTF-8"?> <!-- initially, the default namespace is "books" --> <book xmlns='urn:loc.gov:books' xmlns:isbn='urn:ISBN:0-395-36341-6' xml:lang="en" lang="en"> <title>Cheaper by the Dozen</title> <isbn:number>1568491379</isbn:number> <notes> <!-- make HTML the default namespace for a hypertext commentary --> <p xmlns='http://www.w3.org/1999/xhtml'> This is also available <a href="http://www.w3.org/">online</a>. </p> </notes> </book>
A conforming user agent must meet all of the following criteria:
ID
(i.e. the id
attribute on most XHTML elements) as fragment
identifiers.White space is handled according to the following rules. The following characters are defined in [XML] white space characters:
The XML processor normalizes different systems' line end codes into one single LINE FEED character, that is passed up to the application.
The user agent must use the definition from CSS for processing whitespace characters [CSS2]. Note that the CSS2 recommendation does not explicitly address the issue of whitespace handling in non-Latin character sets. This will be addressed in a future version of CSS, at which time this reference will be updated.
Note that in order to produce a Canonical XHTML document, the rules above must be applied and the rules in [XMLC14N] must also be applied to the document.
This section is informative.
Due to the fact that XHTML is an XML application, certain practices that were perfectly legal in SGML-based HTML 4 [HTML4] must be changed.
Well-formedness is a new concept introduced by [XML]. Essentially this means that all elements must either have closing tags or be written in a special form (as described below), and that all the elements must nest properly.
Although overlapping is illegal in SGML, it is widely tolerated in existing browsers.
CORRECT: nested elements.
<p>here is an emphasized <em>paragraph</em>.</p>
INCORRECT: overlapping elements
<p>here is an emphasized <em>paragraph.</p></em>
XHTML documents must use lower case for all HTML element and attribute names. This difference is necessary because XML is case-sensitive e.g. <li> and <LI> are different tags.
In SGML-based HTML 4 certain elements were permitted to omit the end tag; with the elements that followed implying closure. XML does not allow end tags to be omitted. All elements other than those
declared in the DTD as EMPTY
must have an end tag. Elements that are declared in the DTD as EMPTY
can have an end tag or can use empty element shorthand (see Empty Elements).
CORRECT: terminated elements
<p>here is a paragraph.</p><p>here is another paragraph.</p>
INCORRECT: unterminated elements
<p>here is a paragraph.<p>here is another paragraph.
All attribute values must be quoted, even those which appear to be numeric.
CORRECT: quoted attribute values
<td rowspan="3">
INCORRECT: unquoted attribute values
<td rowspan=3>
XML does not support attribute minimization. Attribute-value pairs must be written in full. Attribute names such as compact
and checked
cannot occur in elements without
their value being specified.
CORRECT: unminimized attributes
<dl compact="compact">
INCORRECT: minimized attributes
<dl compact>
Empty elements must either have an end tag or the start tag must end with />
. For instance, <br/>
or <hr></hr>
. See HTML Compatibility Guidelines for information on ways to ensure this is backward compatible with HTML 4 user agents.
CORRECT: terminated empty elements
<br/><hr/>
INCORRECT: unterminated empty elements
<br><hr>
When user agents process attributes, they do so according to Section 3.3.3 of [XML]:
In XHTML, the script and style elements are declared as having #PCDATA
content. As a result, <
and &
will be treated as the start of markup, and
entities such as <
and &
will be recognized as entity references by the XML processor to <
and &
respectively. Wrapping the
content of the script or style element within a CDATA
marked section avoids the expansion of these entities.
<script type="text/javascript"> <![CDATA[ ... unescaped script content ... ]]> </script>
CDATA
sections are recognized by the XML processor and appear as nodes in the Document Object Model, see Section 1.3 of the DOM Level 1 Recommendation [DOM].
An alternative is to use external script and style documents.
SGML gives the writer of a DTD the ability to exclude specific elements from being contained within an element. Such prohibitions (called "exclusions") are not possible in XML.
For example, the HTML 4 Strict DTD forbids the nesting of an 'a
' element within another 'a
' element to any descendant depth. It is not possible to spell out such
prohibitions in XML. Even though these prohibitions cannot be defined in the DTD, certain elements should not be nested. A summary of such elements and the elements that should not be nested in them
is found in the normative Element Prohibitions.
HTML 4 defined the name
attribute for the elements a
, applet
, form
, frame
, iframe
, img
, and
map
. HTML 4 also introduced the id
attribute. Both of these attributes are designed to be used as fragment identifiers.
In XML, fragment identifiers are of type ID
, and there can only be a single attribute of type ID
per element. Therefore, in XHTML 1.0 the id
attribute is
defined to be of type ID
. In order to ensure that XHTML 1.0 documents are well-structured XML documents, XHTML 1.0 documents MUST use the id
attribute when defining fragment
identifiers on the elements listed above. See the HTML Compatibility Guidelines for information on ensuring such anchors are backward compatible when serving
XHTML documents as media type text/html
.
Note that in XHTML 1.0, the name
attribute of these elements is formally deprecated, and will be removed in a subsequent version of XHTML.
HTML 4 and XHTML both have some attributes that have pre-defined and limited sets of values (e.g. the type
attribute of the input
element). In SGML and XML, these are
called enumerated attributes. Under HTML 4, the interpretation of these values was case-insensitive, so a value of TEXT
was equivalent to a value of text
.
Under XML, the interpretation of these values is case-sensitive, and in XHTML 1 all of these values are defined in lower-case.
SGML and XML both permit references to characters by using hexadecimal values. In SGML these references could be made using either &#Xnn; or &#xnn;. In XML documents, you must use the lower-case version (i.e. &#xnn;)
This section is normative.
Although there is no requirement for XHTML 1.0 documents to be compatible with existing user agents, in practice this is easy to accomplish. Guidelines for creating compatible documents can be found in Appendix C.
XHTML Documents which follow the guidelines set forth in Appendix C, "HTML Compatibility Guidelines" may be labeled with the Internet Media Type "text/html" [RFC2854], as they are compatible with most HTML browsers. Those documents, and any other document conforming to this specification, may also be labeled with the Internet Media Type "application/xhtml+xml" as defined in [RFC3236]. For further information on using media types with XHTML, see the informative note [XHTMLMIME].
This appendix is normative.
These DTDs and entity sets form a normative part of this specification. The complete set of DTD files together with an XML declaration and SGML Open Catalog is included in the zip file and the gzip'd tar file for this specification. Users looking for local copies of the DTDs to work with should download and use those archives rather than using the specific DTDs referenced below.
These DTDs approximate the HTML 4 DTDs. The W3C recommends that you use the authoritative versions of these DTDs at their defined SYSTEM identifiers when validating content. If you need to use these DTDs locally you should download one of the archives of this version. For completeness, the normative versions of the DTDs are included here:
The file DTD/xhtml1-strict.dtd is a normative part of this specification. The annotated contents of this file are available in this separate section for completeness.
The file DTD/xhtml1-transitional.dtd is a normative part of this specification. The annotated contents of this file are available in this separate section for completeness.
The file DTD/xhtml1-frameset.dtd is a normative part of this specification. The annotated contents of this file are available in this separate section for completeness.
The XHTML entity sets are the same as for HTML 4, but have been modified to be valid XML 1.0 entity declarations. Note the entity for the Euro currency sign (€
or
€
or €
) is defined as part of the special characters.
The file DTD/xhtml-lat1.ent is a normative part of this specification. The annotated contents of this file are available in this separate section for completeness.
The file DTD/xhtml-special.ent is a normative part of this specification. The annotated contents of this file are available in this separate section for completeness.
The file DTD/xhtml-symbol.ent is a normative part of this specification. The annotated contents of this file are available in this separate section for completeness.
This appendix is normative.
The following elements have prohibitions on which elements they can contain (see SGML Exclusions). This prohibition applies to all depths of nesting, i.e. it contains all the descendant elements.
a
a
elements.pre
img
, object
, big
, small
, sub
, or sup
elements.button
input
, select
, textarea
, label
, button
, form
, fieldset
, iframe
or
isindex
elements.label
label
elements.form
form
elements.This appendix is informative.
This appendix summarizes design guidelines for authors who wish their XHTML documents to render on existing HTML user agents. Note that this recommendation does not define how HTML conforming
user agents should process HTML documents. Nor does it define the meaning of the Internet Media Type text/html
. For these definitions, see [HTML4] and [RFC2854] respectively.
Be aware that processing instructions are rendered on some user agents. Also, some user agents interpret the XML declaration to mean that the document is unrecognized XML rather than HTML, and therefore may not render the document as expected. For compatibility with these types of legacy browsers, you may want to avoid using processing instructions and XML declarations. Remember, however, that when the XML declaration is not included in a document, the document can only use the default character encodings UTF-8 or UTF-16.
Include a space before the trailing /
and >
of empty elements, e.g. <br />
, <hr />
and <img src="karen.jpg" alt="Karen" />
. Also, use the minimized tag syntax for empty elements, e.g. <br />
, as the
alternative syntax <br></br>
allowed by XML gives uncertain results in many existing user agents.
Given an empty instance of an element whose content model is not EMPTY
(for example, an empty title or paragraph) do not use the minimized form (e.g. use
<p> </p>
and not <p />
).
Use external style sheets if your style sheet uses <
or &
or ]]>
or --
. Use external scripts if your script uses <
or
&
or ]]>
or --
. Note that 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 is likely to not work as expected in XML-based user agents.
Avoid line breaks and multiple white space characters within attribute values. These are handled inconsistently by user agents.
Don't include more than one isindex
element in the document head
. The isindex
element is deprecated in favor of the input
element.
lang
and xml:lang
AttributesUse both the lang
and xml:lang
attributes when specifying the language of an element. The value of the xml:lang
attribute takes precedence.
In XML, URI-references [RFC2396] that end with fragment identifiers of the form
"#foo"
do not refer to elements with an attribute name="foo"
; rather, they refer to elements with an attribute defined to be of type ID
, e.g., the id
attribute in HTML 4. Many existing HTML clients don't support the use of ID
-type attributes in this way, so identical values may be supplied for both of these attributes to ensure
maximum forward and backward compatibility (e.g., <a id="foo" name="foo">...</a>
).
Further, since the set of legal values for attributes of type ID
is much smaller than for those of type CDATA
, the type of the name
attribute has been
changed to NMTOKEN
. This attribute is constrained such that it can only have the same values as type ID
, or as the Name
production in XML 1.0 Section 2.3,
production 5. Unfortunately, this constraint cannot be expressed in the XHTML 1.0 DTDs. Because of this change, care must be taken when converting existing HTML documents. The values of these
attributes must be unique within the document, valid, and any references to these fragment identifiers (both internal and external) must be updated should the values be changed during conversion.
Note that the collection of legal values in XML 1.0 Section 2.3, production 5 is much larger than that permitted to be used in the ID
and NAME
types defined in HTML 4.
When defining fragment identifiers to be backward-compatible, only strings matching the pattern [A-Za-z][A-Za-z0-9:_.-]*
should be used. See Section 6.2 of [HTML4] for more information.
Finally, note that XHTML 1.0 has deprecated the name
attribute of the a
, applet
, form
, frame
, iframe
,
img
, and map
elements, and it will be removed from XHTML in subsequent versions.
Historically, the character encoding of an HTML document is either specified by a web server via the charset parameter of the HTTP Content-Type header, or via a meta
element in the
document itself. In an XML document, the character encoding of the document is specified on the XML declaration (e.g., <?xml version="1.0" encoding="EUC-JP"?>
).
In order to portably present documents with specific character encodings, the best approach is to ensure that the web server provides the correct headers. If this is not possible, a document that
wants to set its character encoding explicitly must include both the XML declaration an encoding declaration and a meta
http-equiv statement (e.g., <meta
http-equiv="Content-type" content="text/html; charset=EUC-JP" />
). In XHTML-conforming user agents, the value of the encoding declaration of the XML declaration takes precedence.
Note: be aware that if a document must include the character encoding declaration in a meta http-equiv statement, that document may always be interpreted by HTTP servers and/or user agents as being of the internet media type defined in that statement. If a document is to be served as multiple media types, the HTTP server must be used to set the encoding of the document.
Some HTML user agents are unable to interpret boolean attributes when these appear in their full (non-minimized) form, as required by XML 1.0. Note this problem doesn't affect user agents
compliant with HTML 4. The following attributes are involved: compact
, nowrap
, ismap
, declare
, noshade
, checked
,
disabled
, readonly
, multiple
, selected
, noresize
, defer
.
The Document Object Model level 1 Recommendation [DOM] defines document object model interfaces for XML and HTML 4. The HTML 4 document object model specifies that HTML element and attribute names are returned in upper-case. The XML document object model specifies that element and attribute names are returned in the case they are specified. In XHTML 1.0, elements and attributes are specified in lower-case. This apparent difference can be addressed in two ways:
text/html
via the DOM can use the HTML DOM, and can rely upon element
and attribute names being returned in upper-case from those interfaces.text/xml
, application/xml
, or application/xhtml+xml
can also use the XML DOM.
Elements and attributes will be returned in lower-case. Also, some XHTML elements may or may not appear in the object tree because they are optional in the content model (e.g. the tbody
element within table
). This occurs because in HTML 4 some elements were permitted to be minimized such that their start and end tags are both omitted (an SGML feature). This is not
possible in XML. Rather than require document authors to insert extraneous elements, XHTML has made the elements optional. User agents need to adapt to this accordingly. For further information on
this topic, see [DOM2]In both SGML and XML, the ampersand character ("&") declares the beginning of an entity reference (e.g., ® for the registered trademark symbol "®"). Unfortunately, many HTML user
agents have silently ignored incorrect usage of the ampersand character in HTML documents - treating ampersands that do not look like entity references as literal ampersands. XML-based user agents
will not tolerate this incorrect usage, and any document that uses an ampersand incorrectly will not be "valid", and consequently will not conform to this specification. In order to ensure that
documents are compatible with historical HTML user agents and XML-based user agents, ampersands used in a document that are to be treated as literal characters must be expressed themselves as an
entity reference (e.g. "&
"). For example, when the href
attribute of the a
element refers to a CGI script that takes parameters, it must be expressed as
http://my.site.dom/cgi-bin/myscript.pl?class=guest&name=user
rather than as http://my.site.dom/cgi-bin/myscript.pl?class=guest&name=user
.
The Cascading Style Sheets level 2 Recommendation [CSS2] defines style properties which are applied to the parse tree of the HTML or XML documents. Differences in parsing will produce different visual or aural results, depending on the selectors used. The following hints will reduce this effect for documents which are served without modification as both media types:
In HTML 4 and XHTML, the style
element can be used to define document-internal style rules. In XML, an XML stylesheet declaration is used to define style rules. In order to be
compatible with this convention, style
elements should have their fragment identifier set using the id
attribute, and an XML stylesheet declaration should reference this
fragment. For example:
<?xml-stylesheet href="http://www.w3.org/StyleSheets/TR/W3C-REC.css" type="text/css"?> <?xml-stylesheet href="#internalStyle" type="text/css"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <title>An internal stylesheet example</title> <style type="text/css" id="internalStyle"> code { color: green; font-family: monospace; font-weight: bold; } </style> </head> <body> <p> This is text that uses our <code>internal stylesheet</code>. </p> </body> </html>
Some characters that are legal in HTML documents, are illegal in XML document. For example, in HTML, the Formfeed character (U+000C) is treated as white space, in XHTML, due to XML's definition of characters, it is illegal.
The named character reference '
(the apostrophe, U+0027) was introduced in XML 1.0 but does not appear in HTML. Authors should therefore use '
instead of
'
to work as expected in HTML 4 user agents.
This appendix is informative.
This specification was written with the participation of the members of the W3C HTML Working Group.
At publication of the second edition, the membership was:
At publication of the first edition, the membership was:
This appendix is informative.