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W3C

XHTML™ 1.0 The Extensible HyperText Markup Language (Second Edition)

A Reformulation of HTML 4 in XML 1.0

W3C Recommendation 26 January 2000, revised 1 August 2002
superseded 27 March 2018

This version:
http://www.w3.org/TR/2018/SPSD-xhtml1-20180327/
Latest version:
http://www.w3.org/TR/xhtml1
Previous version:
http://www.w3.org/TR/2002/REC-xhtml1-20020801
Authors:
Seeacknowledgments.

Please refer to theerrata for this document, which may include some normative corrections. See alsotranslations.

This document is also available in these non-normative formats:Multi-part XHTML file,PostScript version,PDF version,ZIP archive, andGzip'd TAR archive.

Copyright ©2002W3C® (MIT,INRIA,Keio), All Rights Reserved. W3Cliability,trademark,document use andsoftware licensing rules apply.

Abstract

This specification defines the Second Edition ofXHTML 1.0, a reformulation of HTML 4 as anXML 1.0 application, and threeDTDs corresponding to the ones defined by HTML 4. The semantics of the elements and their attributes aredefined 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 asmall set of guidelines.

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 theW3C.

This specification is aSuperseded Recommendation. A newerspecification exists that is recommended for new adoption in place ofthis specification. New implementations should follow thelatest version of the HTML specification.

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 areillustrated in adiff-marked version.

This second edition isnot 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 thecommunity 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.

This document has been produced as part of theW3C HTML Activity.

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 befound on the Working Group'spatent disclosure page.

A list of current W3C Recommendations and other technical documentscan be found athttps://www.w3.org/TR/.

Quick Table of Contents

Full Table of Contents

1. What is XHTML?

This section is informative.

XHTML is a family of current and future document types and modules that reproduce, subset, and extend HTML 4 [HTML4]. XHTML familydocument types areXML based, and ultimately are designed to work in conjunction with XML-based user agents. The details of this family and itsevolution 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 simpleguidelines arefollowed, 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 stillremaining confident in their content's backward and future compatibility.

1.1. What is HTML 4?

HTML 4 [HTML4] is anSGML (Standard Generalized Markup Language) applicationconforming to International StandardISO 8879, and is widely regarded as the standard publishing language of the World WideWeb.

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 definedin 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 SGMLcomplexity 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 forhypertext. 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.

1.2. What is XML?

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 andrichness, 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.

1.3. Why the need for XHTML?

The benefits of migrating to XHTML 1.0 are described above. Some of the benefits of migrating to XHTML in general are:

2. Definitions

This section is normative.

2.1. Terminology

The following terms are used in this specification. These terms extend the definitions in [RFC2119] in ways based upon similar definitions inISO/IEC 9945-1:1990 [POSIX.1]:

May
With respect to implementations, the word "may" is to be interpreted as an optional feature that is not required in this specification but can be provided. With respect toDocument Conformance, the word "may" means that the optional feature must not be used. The term "optional" has the same definition as "may".
Must
In this specification, the word "must" is to be interpreted as a mandatory requirement on the implementation or on Strictly Conforming XHTML Documents, depending upon the context. The term"shall" has the same definition as "must".
Optional
See "May".
Reserved
A value or behavior is unspecified, but it is not allowed to be used by Conforming Documents nor to be supported by Conforming User Agents.
Shall
See "Must".
Should
With respect to implementations, the word "should" is to be interpreted as an implementation recommendation, but not a requirement. With respect to documents, the word "should" is to beinterpreted as recommended programming practice for documents and a requirement for Strictly Conforming XHTML Documents.
Supported
Certain facilities in this specification are optional. If a facility is supported, it behaves as specified by this specification.
Unspecified
When a value or behavior is unspecified, the specification defines no portability requirements for a facility on an implementation even when faced with a document that uses the facility. Adocument that requires specific behavior in such an instance, rather than tolerating any behavior when using that facility, is not a Strictly Conforming XHTML Document.

2.2. General Terms

Attribute
An attribute is a parameter to an element declared in the DTD. An attribute's type and value range, including a possible default value, are defined in the DTD.
DTD
A DTD, or document type definition, is a collection of XML markup declarations that, as a collection, defines the legal structure,elements, andattributes that are available for use in a document that complies to the DTD.
Document
A document is a stream of data that, after being combined with any other streams it references, is structured such that it holds information contained withinelementsthat are organized as defined in the associatedDTD. SeeDocument Conformance for more information.
Element
An element is a document structuring unit declared in theDTD. The element's content model is defined in theDTD, and additional semantics maybe defined in the prose description of the element.
Facilities
Facilities areelements,attributes, and the semantics associated with thoseelements andattributes.
Implementation
See User Agent.
Parsing
Parsing is the act whereby adocument is scanned, and the information contained within thedocument is filtered into the context of theelements in which the information is structured.
Rendering
Rendering is the act whereby the information in adocument is presented. This presentation is done in the form most appropriate to the environment (e.g. aurally,visually, in print).
User Agent
A user agent is a system that processes XHTML documents in accordance with this specification. SeeUser Agent Conformance for more information.
Validation
Validation is a process wherebydocuments are verified against the associatedDTD, ensuring that the structure, use ofelements, and use ofattributes are consistent with the definitions in theDTD.
Well-formed
Adocument is well-formed when it is structured according to the rules defined inSection 2.1 of the XML 1.0Recommendation [XML].

3. Normative Definition of XHTML 1.0

This section is normative.

3.1. Document Conformance

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. SeeSection 3.1.2 for information on using XHTML with other namespaces, for instance, to include metadata expressed inRDF within XHTMLdocuments.

3.1.1. Strictly Conforming 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:

  1. It must conform to the constraints expressed in one of the three DTDs found inDTDs and inAppendix B.

  2. The root element of the document must behtml.

  3. The root element of the document must contain anxmlns declaration for the XHTML namespace [XMLNS]. The namespace for XHTML isdefined to behttp://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">

  4. 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 inDTDs 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">

  5. 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 whenthe 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>

3.1.2. Using XHTML with other namespaces

The XHTML namespace may be used with other XML namespaces as per [XMLNS], although such documents are not strictly conforming XHTML 1.0documents 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>

3.2. User Agent Conformance

A conforming user agent must meet all of the following criteria:

  1. In order to be consistent with the XML 1.0 Recommendation [XML], the user agent must parse and evaluate an XHTML document for well-formedness.If the user agent claims to be a validating user agent, it must also validate documents against their referenced DTDs according to [XML].
  2. When the user agent claims to supportfacilities defined within this specification or required by this specification through normative reference, it mustdo so in ways consistent with the facilities' definition.
  3. When a user agent processes an XHTML document as generic XML, it shall only recognize attributes of typeID (i.e. theid attribute on most XHTML elements) as fragmentidentifiers.
  4. If a user agent encounters an element it does not recognize, it must process the element's content.
  5. If a user agent encounters an attribute it does not recognize, it must ignore the entire attribute specification (i.e., the attribute and its value).
  6. If a user agent encounters an attribute value it does not recognize, it must use the default attribute value.
  7. If it encounters an entity reference (other than one of the entities defined in this recommendation or in the XML recommendation) for which the user agent has processed no declaration (whichcould happen if the declaration is in the external subset which the user agent hasn't read), the entity reference should be processed as the characters (starting with the ampersand and ending withthe semi-colon) that make up the entity reference.
  8. When processing content, user agents that encounter characters or character entity references that are recognized but not renderable may substitute another rendering that gives the same meaning,or must display the document in such a way that it is obvious to the user that normal rendering has not taken place.

  9. 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 notexplicitly 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 tothe document.

4. Differences with HTML 4

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 bechanged.

4.1. Documents must be well-formed

Well-formedness is a new concept introduced by [XML]. Essentially this means that all elements musteither 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>

4.2. Element and attribute names must be in lower case

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.

4.3. For non-empty elements, end tags are required

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 thosedeclared in the DTD asEMPTY must have an end tag. Elements that are declared in the DTD asEMPTY can have an end tagor can use empty element shorthand (seeEmpty 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.

4.4. Attribute values must always be quoted

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>

4.5. Attribute Minimization

XML does not support attribute minimization. Attribute-value pairs must be written in full. Attribute names such ascompact andchecked cannot occur in elements withouttheir value being specified.

CORRECT: unminimized attributes

<dl compact="compact">

INCORRECT: minimized attributes

<dl compact>

4.6. Empty Elements

Empty elements must either have an end tag or the start tag must end with/>. For instance,<br/> or<hr></hr>. SeeHTML 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>

4.7. White Space handling in attribute values

When user agents process attributes, they do so according toSection 3.3.3 of [XML]:

4.8. Script and Style elements

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, andentities such as&lt; and&amp; will be recognized as entity references by the XML processor to< and& respectively. Wrapping thecontent of the script or style element within aCDATA 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, seeSection 1.3 of the DOM Level 1 Recommendation [DOM].

An alternative is to use external script and style documents.

4.9. SGML exclusions

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 suchprohibitions 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 themis found in the normativeElement Prohibitions.

4.10. The elements with 'id' and 'name' attributes

HTML 4 defined thename attribute for the elementsa,applet,form,frame,iframe,img, andmap. HTML 4 also introduced theid attribute. Both of these attributes are designed to be used as fragment identifiers.

In XML, fragment identifiers are of typeID, and there can only be a single attribute of typeID per element. Therefore, in XHTML 1.0 theid attribute isdefined to be of typeID. In order to ensure that XHTML 1.0 documents are well-structured XML documents, XHTML 1.0 documents MUST use theid attribute when defining fragmentidentifiers on the elements listed above. See theHTML Compatibility Guidelines for information on ensuring such anchors are backward compatible when servingXHTML documents as media typetext/html.

Note that in XHTML 1.0, thename attribute of these elements is formally deprecated, and will be removed in a subsequent version of XHTML.

4.11. Attributes with pre-defined value sets

HTML 4 and XHTML both have some attributes that have pre-defined and limited sets of values (e.g. thetype attribute of theinput element). In SGML and XML, these arecalledenumerated attributes. Under HTML 4, the interpretation of these values wascase-insensitive, so a value ofTEXT was equivalent to a value oftext.Under XML, the interpretation of these values iscase-sensitive, and in XHTML 1 all of these values are defined in lower-case.

4.12. Entity references as hex values

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 thelower-case version (i.e. &#xnn;)

5. Compatibility Issues

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 befound inAppendix C.

5.1. Internet Media Type

XHTML Documents which follow the guidelines set forth inAppendix 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 typeswith XHTML, see the informative note [XHTMLMIME].

A. DTDs

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 thezip file and thegzip'd tar file for this specification. Users looking for local copies of the DTDs to work with should download and use those archivesrather than using the specific DTDs referenced below.

A.1. Document Type Definitions

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 usethese DTDs locally you should download one of the archives ofthis version. For completeness, the normative versions of the DTDs are included here:

A.1.1. XHTML-1.0-Strict

The fileDTD/xhtml1-strict.dtd is a normative part of this specification. The annotated contents of this file are available in thisseparate section for completeness.

A.1.2. XHTML-1.0-Transitional

The fileDTD/xhtml1-transitional.dtd is a normative part of this specification. The annotated contents of this file are available in thisseparate section for completeness.

A.1.3. XHTML-1.0-Frameset

The fileDTD/xhtml1-frameset.dtd is a normative part of this specification. The annotated contents of this file are available in thisseparate section for completeness.

A.2. Entity Sets

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 (&euro; or&#8364; or&#x20AC;) is defined as part of the special characters.

A.2.1. Latin-1 characters

The fileDTD/xhtml-lat1.ent is a normative part of this specification. The annotated contents of this file are available in thisseparate section for completeness.

A.2.2. Special characters

The fileDTD/xhtml-special.ent is a normative part of this specification. The annotated contents of this file are available in thisseparate section for completeness.

A.2.3. Symbols

The fileDTD/xhtml-symbol.ent is a normative part of this specification. The annotated contents of this file are available in thisseparate section for completeness.

B. Element Prohibitions

This appendix is normative.

The following elements have prohibitions on which elements they can contain (seeSGML Exclusions). This prohibition applies to all depths of nesting, i.e. itcontains all the descendant elements.

a
must not contain othera elements.
pre
must not contain theimg,object,big,small,sub, orsup elements.
button
must not contain theinput,select,textarea,label,button,form,fieldset,iframe orisindex elements.
label
must not contain otherlabel elements.
form
must not contain otherform elements.

C. HTML Compatibility Guidelines

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 conforminguser agents should process HTML documents. Nor does it define the meaning of the Internet Media Typetext/html. For these definitions, see [HTML4] and [RFC2854] respectively.

C.1. Processing Instructions and the XML Declaration

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, andtherefore 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.

C.2. Empty Elements

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 thealternative syntax<br></br> allowed by XML gives uncertain results in many existing user agents.

C.3. Element Minimization and Empty Element Content

Given an empty instance of an element whose content model is notEMPTY (for example, an empty title or paragraph) do not use the minimized form (e.g. use<p> </p> and not<p />).

C.4. Embedded Style Sheets and Scripts

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" scriptsand style sheets within "comments" to make the documents backward compatible is likely to not work as expected in XML-based user agents.

C.5. Line Breaks within Attribute Values

Avoid line breaks and multiple white space characters within attribute values. These are handled inconsistently by user agents.

C.6. Isindex

Don't include more than oneisindex element in the documenthead. Theisindex element is deprecated in favor of theinput element.

C.7. Thelang andxml:lang Attributes

Use both thelang andxml:lang attributes when specifying the language of an element. The value of thexml:lang attribute takes precedence.

C.8. Fragment Identifiers

In XML,URI-references [RFC2396] that end with fragment identifiers of the form"#foo" do not refer to elements with an attributename="foo"; rather, they refer to elements with an attribute defined to be of typeID, e.g., theidattribute in HTML 4. Many existing HTML clients don't support the use ofID-type attributes in this way, so identical values may be supplied for both of these attributes to ensuremaximum forward and backward compatibility (e.g.,<a name="foo">...</a>).

Further, since the set of legal values for attributes of typeID is much smaller than for those of typeCDATA, the type of thename attribute has beenchanged toNMTOKEN. This attribute is constrained such that it can only have the same values as typeID, or as theName 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 theseattributes 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 theID andNAME 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. SeeSection 6.2 of [HTML4] for more information.

Finally, note that XHTML 1.0 has deprecated thename attribute of thea,applet,form,frame,iframe,img, andmap elements, and it will be removed from XHTML in subsequent versions.

C.9. Character Encoding

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 ameta element in thedocument 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 thatwants to set its character encoding explicitly must include both the XML declaration an encoding declaration and ameta http-equiv statement (e.g.,<metahttp-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 asbeing 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.

C.10. Boolean Attributes

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 agentscompliant with HTML 4. The following attributes are involved:compact,nowrap,ismap,declare,noshade,checked,disabled,readonly,multiple,selected,noresize,defer.

C.11. Document Object Model and XHTML

The Document Object Model level 1 Recommendation [DOM] defines document object model interfaces for XML and HTML 4. The HTML 4 document objectmodel 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:

  1. User agents that access XHTML documents served as Internet media typetext/html via theDOM can use the HTML DOM, and can rely upon elementand attribute names being returned in upper-case from those interfaces.
  2. User agents that access XHTML documents served as Internet media typestext/xml,application/xml, orapplication/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. thetbodyelement withintable). 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 notpossible 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 onthis topic, see [DOM2]

C.12. Using Ampersands in Attribute Values (and Elsewhere)

In both SGML and XML, the ampersand character ("&") declares the beginning of an entity reference (e.g., &reg; for the registered trademark symbol "®"). Unfortunately, many HTML useragents 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 agentswill 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 thatdocuments 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 anentity reference (e.g. "&amp;"). For example, when thehref attribute of thea element refers to a CGI script that takes parameters, it must be expressed ashttp://my.site.dom/cgi-bin/myscript.pl?class=guest&amp;name=user rather than ashttp://my.site.dom/cgi-bin/myscript.pl?class=guest&name=user.

C.13. Cascading Style Sheets (CSS) and XHTML

The Cascading Style Sheets level 2 Recommendation [CSS2] defines style properties which are applied to the parse tree of the HTML or XMLdocuments. 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 withoutmodification as both media types:

  1. CSS style sheets for XHTML should use lower case element and attribute names.
  2. In tables, the tbody element will be inferred by the parser of an HTML user agent, but not by the parser of an XML user agent. Therefore you should always explicitly add a tbody element if it isreferred to in a CSS selector.
  3. Within the XHTML namespace, user agents are expected to recognize the "id" attribute as an attribute of type ID. Therefore, style sheets should be able to continue using the shorthand "#"selector syntax even if the user agent does not read the DTD.
  4. Within the XHTML namespace, user agents are expected to recognize the "class" attribute. Therefore, style sheets should be able to continue using the shorthand "." selector syntax.
  5. CSS defines different conformance rules for HTML and XML documents; be aware that the HTML rules apply to XHTML documents delivered as HTML and the XML rules apply to XHTML documents delivered asXML.

C.14. Referencing Style Elements when serving as XML

In HTML 4 and XHTML, thestyle 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 becompatible with this convention,style elements should have their fragment identifier set using theid attribute, and an XML stylesheet declaration should reference thisfragment. 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">  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>

C.15. White Space Characters in HTML vs. XML

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 ofcharacters, it is illegal.

C.16. The Named Character Reference &apos;

The named character reference&apos; (the apostrophe, U+0027) was introduced in XML 1.0 but does not appear in HTML. Authors should therefore use&#39; instead of&apos; to work as expected in HTML 4 user agents.

D. Acknowledgements

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:

Steven Pemberton, CWI/W3C (HTML Working Group Chair)
Daniel Austin, Grainger
Jonny Axelsson, Opera Software
Tantek Çelik, Microsoft
Doug Dominiak, Openwave Systems
Herman Elenbaas, Philips Electronics
Beth Epperson, Netscape/AOL
Masayasu Ishikawa, W3C (HTML Activity Lead)
Shin'ichi Matsui, Panasonic
Shane McCarron, Applied Testing and Technology
Ann Navarro, WebGeek,Inc.
Subramanian Peruvemba, Oracle
Rob Relyea, Microsoft
Sebastian Schnitzenbaumer, SAP
Peter Stark, Sony Ericsson

At publication of the first edition, the membership was:

Steven Pemberton,CWI (HTML Working Group Chair)
Murray Altheim, Sun Microsystems
Daniel Austin, AskJeeves (CNET: The Computer Network through July 1999)
Frank Boumphrey, HTML Writers Guild
John Burger, Mitre
Andrew W. Donoho, IBM
Sam Dooley, IBM
Klaus Hofrichter, GMD
Philipp Hoschka, W3C
Masayasu Ishikawa, W3C
Warner ten Kate, Philips Electronics
Peter King, Phone.com
Paula Klante, JetForm
Shin'ichi Matsui, Panasonic (W3C visiting engineer through September 1999)
Shane McCarron, Applied Testing and Technology (The Open Group through August 1999)
Ann Navarro, HTML Writers Guild
Zach Nies, Quark
Dave Raggett, W3C/HP (HTML Activity Lead)
Patrick Schmitz, Microsoft
Sebastian Schnitzenbaumer, Stack Overflow
Peter Stark, Phone.com
Chris Wilson, Microsoft
Ted Wugofski, Gateway 2000
Dan Zigmond, WebTV Networks

E. References

This appendix is informative.

[CSS2]
"Cascading Style Sheets, level 2 (CSS2) Specification", B. Bos, H. W. Lie, C. Lilley, I. Jacobs, 12 May 1998.
Latest version available at: http://www.w3.org/TR/REC-CSS2
[DOM]
"Document Object Model (DOM) Level 1 Specification", Lauren Woodet al., 1 October1998.
Latest version available at: http://www.w3.org/TR/REC-DOM-Level-1
[DOM2]
"Document Object Model (DOM) Level 2 Core Specification", A. Le Hors,etal., 13 November 2000.
Latest version available at: http://www.w3.org/TR/DOM-Level-2-Core
[HTML]
"HTML 4.01 Specification", D. Raggett, A. Le Hors, I. Jacobs, 24 December 1999.
Latest version available at: http://www.w3.org/TR/html401
[POSIX.1]
"ISO/IEC 9945-1:1990 Information Technology - Portable Operating System Interface (POSIX) - Part 1: System Application Program Interface (API) [C Language]", Institute of Electricaland Electronics Engineers, Inc, 1990.
[RFC2045]
"Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", N. Freed and N. Borenstein, November1996. Note that this RFC obsoletes RFC1521, RFC1522, and RFC1590.
[RFC2046]
"RFC2046: Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", N. Freed and N. Borenstein, November 1996.
Available athttp://www.ietf.org/rfc/rfc2046.txt. Note that this RFC obsoletes RFC1521, RFC1522, and RFC1590.
[RFC2119]
"RFC2119: Key words for use in RFCs to Indicate Requirement Levels", S. Bradner, March 1997.
Available at: http://www.ietf.org/rfc/rfc2119.txt
[RFC2376]
"RFC2376: XML Media Types", E. Whitehead, M. Murata, July 1998.
This document is obsoleted by [RFC3023].
Available at: http://www.ietf.org/rfc/rfc2376.txt
[RFC2396]
"RFC2396: Uniform Resource Identifiers (URI): Generic Syntax", T. Berners-Lee, R. Fielding, L. Masinter, August 1998.
This document updates RFC1738 and RFC1808.
Available at: http://www.ietf.org/rfc/rfc2396.txt
[RFC2854]
"RFC2854: The text/html Media Type", D. Conolly, L. Masinter, June 2000.
Available at: http://www.ietf.org/rfc/rfc2854.txt
[RFC3023]
"RFC3023: XML Media Types", M. Murata, S. St.Laurent, D. Kohn, January 2001.
This document obsoletes [RFC2376].
Available at: http://www.ietf.org/rfc/rfc3023.txt
[RFC3066]
"Tags for the Identification of Languages", H. Alvestrand, January 2001.
Available at: http://www.ietf.org/rfc/rfc3066.txt
[RFC3236]
"The 'application/xhtml+xml' Media Type", M. Baker, P. Stark, January 2002.
Available at: http://www.ietf.org/rfc/rfc3236.txt
[XHTML+MathML]
"XHTML plus Math 1.1DTD", "A.2 MathML as a DTD Module", MathematicalMarkup Language (MathML) Version 2.0. Available at: http://www.w3.org/TR/MathML2/dtd/xhtml-math11-f.dtd
[XHTMLMIME]
"XHTML Media Types", Masayasu Ishikawa, 1 August 2002.
Latest version available at: http://www.w3.org/TR/xhtml-media-types
[XHTMLMOD]
"Modularization of XHTML", M. Altheim et al., 10 April 2001.
Latest version available at: http://www.w3.org/TR/xhtml-modularization
[XML]
"Extensible Markup Language (XML) 1.0 Specification (Second Edition)", T. Bray, J. Paoli, C. M. Sperberg-McQueen, E. Maler, 6 October2000.
Latest version available at: http://www.w3.org/TR/REC-xml
[XMLNS]
"Namespaces in XML", T. Bray, D. Hollander, A. Layman, 14 January 1999.
XML namespaces provide a simple method for qualifying names used in XML documents by associating them with namespaces identified by URI.
Latest version available at: http://www.w3.org/TR/REC-xml-names
[XMLC14N]
"Canonical XML Version 1.0", J. Boyer, 15 March 2001.
This document describes a method for generating a physical representation, the canonical form, of an XML document.
Latest version available at: http://www.w3.org/TR/xml-c14n

Level Triple-A conformance icon, W3C-WAI Web Content Accessibility Guidelines 1.0

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