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XHTML™ 1.0 The Extensible HyperText Markup Language (2nd Edition)

A Reformulation of HTML four in XML i.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superseded 27 March 2018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:

See acknowledgments.

Delight 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-office 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.

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Abstract

This specification defines the Second Edition of XHTML i.0, a reformulation of HTML four equally an XML i.0 awarding, and three DTDs respective to the ones defined past 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 time to come extensibility of XHTML. Compatibility with existing HTML user agents is possible by following a pocket-size set of guidelines.

Status of this certificate

This section describes the status of this document at the time of its publication. Other documents may supersede this certificate. The latest status of this document series is maintained at the W3C.

This specification is a Superseded Recommendation. A newer specification exists that is recommended for new adoption in place of this specification. New implementations should follow the latest version of the HTML specification.

This document is the second edition of the XHTML 1.0 specification incorporating the errata changes as of 1 Baronial 2002. Changes betwixt this version and the previous Recommendation are illustrated in a diff-marked version.

This 2d edition is non a new version of XHTML one.0 (first published 26 January 2000). The changes in this document reflect corrections practical every bit a result of comments submitted past the community and as a result of ongoing piece of work within the HTML Working Grouping. In that location are no substantive changes in this document - only the integration of various errata.

This document has been produced as function of the W3C HTML Activeness.

At the time of publication, the working group believed there were cypher patent disclosures relevant to this specification. A electric current list of patent disclosures relevant to this specification may exist found on the Working Grouping'southward patent disclosure page.

A list of current W3C Recommendations and other technical documents can be found at https://www.w3.org/TR/.

Quick Tabular array of Contents

• 1. What is XHTML?
• 2. Definitions
• iii. Normative Definition of XHTML 1.0
• 4. Differences with HTML iv
• 5. Compatibility Issues
• A. DTDs
• B. Element Prohibitions
• C. HTML Compatibility Guidelines
• D. Acknowledgements
• Eastward. References

Full Table of Contents

• ane. What is XHTML?
  • one.1. What is HTML iv?
  • ane.two. What is XML?
  • one.3. Why the need for XHTML?
• two. Definitions
  • 2.1. Terminology
  • 2.2. Full general Terms
• 3. Normative Definition of XHTML 1.0
  • 3.one. Document Conformance
    • 3.1.one. Strictly Conforming Documents
    • 3.1.2. Using XHTML with other namespaces
  • 3.2. User Agent Conformance
• 4. Differences with HTML iv
  • four.one. Documents must be well-formed
  • 4.2. Chemical element and attribute names must be in lower case
  • 4.3. For not-empty elements, end tags are required
  • 4.4. Aspect values must e'er be quoted
  • 4.5. Attribute Minimization
  • 4.six. Empty Elements
  • four.7. White Infinite treatment in attribute values
  • four.8. Script and Manner elements
  • 4.ix. SGML exclusions
  • iv.10. The elements with 'id' and 'proper noun' attributes
  • iv.11. Attributes with pre-defined value sets
  • 4.12. Entity references as hex values
• v. Compatibility Issues
  • 5.i. Internet Media Blazon
• A. DTDs
  • A.1. Document Type Definitions
    • A.1.1. XHTML-i.0-Strict
    • A.ane.2. XHTML-1.0-Transitional
    • A.1.3. XHTML-1.0-Frameset
  • A.2. Entity Sets
    • A.2.1. Latin-1 characters
    • A.2.2. Special characters
    • A.2.3. Symbols
• B. Element Prohibitions
• C. HTML Compatibility Guidelines
  • C.i. Processing Instructions and the XML Declaration
  • C.2. Empty Elements
  • C.iii. Element Minimization and Empty Chemical element Content
  • C.4. Embedded Style Sheets and Scripts
  • C.5. Line Breaks within Attribute Values
  • C.vi. Isindex
  • C.7. The lang and xml:lang Attributes
  • C.8. Fragment Identifiers
  • C.9. Graphic symbol Encoding
  • C.10. Boolean Attributes
  • C.11. Document Object Model and XHTML
  • C.12. Using Ampersands in Attribute Values (and Elsewhere)
  • C.13. Cascading Style Sheets (CSS) and XHTML
  • C.fourteen. Referencing Style Elements when serving every bit XML
  • C.15. White Space Characters in HTML vs. XML
  • C.16. The Named Character Reference '
• D. Acknowledgements
• Eastward. References

1. What is XHTML?

This section is informative.

XHTML is a family of electric 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 unit and its development are discussed in more particular in [XHTMLMOD].

XHTML 1.0 (this specification) is the starting time document type in the XHTML family. Information technology is a reformulation of the three HTML 4 certificate types as applications of XML one.0 [XML]. It is intended to be used as a language for content that is both XML-conforming and, if some unproblematic guidelines are followed, operates in HTML 4 conforming user agents. Developers who migrate their content to XHTML ane.0 volition realize the following benefits:

• XHTML documents are XML befitting. Equally such, they are readily viewed, edited, and validated with standard XML tools.
• XHTML documents can exist written to operate as well or better than they did before in existing HTML four-conforming user agents also equally in new, XHTML 1.0 conforming user agents.
• XHTML documents can utilize applications (e.grand. scripts and applets) that rely upon either the HTML Document Object Model or the XML Document Object Model [DOM].
• As the XHTML family evolves, documents befitting to XHTML one.0 will be more likely to interoperate within and among various XHTML environments.

The XHTML family is the side by side step in the evolution of the Internet. By migrating to XHTML today, content developers tin enter the XML globe with all of its attendant benefits, while still remaining confident in their content'due south backward and future compatibility.

ane.1. What is HTML iv?

HTML iv [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, peculiarly those used in electronic document exchange, document management, and certificate publishing. HTML is an example of a language divers 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 linguistic communication is both feature-rich and flexible. This flexibility, however, comes at a toll, and that price is a level of complication that has inhibited its adoption in a diverseness of environments, including the World wide web.

HTML, every bit originally conceived, was to be a language for the commutation of scientific and other technical documents, suitable for utilize 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 certificate structure, HTML added support for hypertext. Multimedia capabilities were added subsequently.

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 employ 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.

ane.2. What is XML?

XML™ is the shorthand name for Extensible Markup Language [XML].

XML was conceived equally 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 withal retains all of SGML's ordinarily used features.

While retaining these beneficial features, XML removes many of the more complex features of SGML that brand the authoring and design of suitable software both hard and plush.

ane.3. Why the demand for XHTML?

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

• Certificate developers and user amanuensis designers are constantly discovering new ways to express their ideas through new markup. In XML, information technology is relatively easy to introduce new elements or additional chemical element attributes. The XHTML family is designed to accommodate these extensions through XHTML modules and techniques for developing new XHTML-conforming modules (described in the XHTML Modularization specification). These modules will permit the combination of existing and new feature sets when developing content and when designing new user agents.
• Alternate ways of accessing the Internet are constantly being introduced. The XHTML family is designed with general user agent interoperability in mind. Through a new user agent and document profiling machinery, servers, proxies, and user agents will exist able to perform best attempt content transformation. Ultimately, it will be possible to develop XHTML-conforming content that is usable past any XHTML-conforming user amanuensis.

two. 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 in ISO/IEC 9945-1:1990 [POSIX.1]:

May

With respect to implementations, the word "may" is to be interpreted equally an optional feature that is not required in this specification but can be provided. With respect to Certificate Conformance, the word "may" means that the optional characteristic must non 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 every bit "must".

Optional

See "May".

Reserved

A value or behavior is unspecified, only it is non allowed to be used past 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 every bit an implementation recommendation, merely not a requirement. With respect to documents, the word "should" is to be interpreted as recommended programming do for documents and a requirement for Strictly Conforming XHTML Documents.

Supported

Certain facilities in this specification are optional. If a facility is supported, information technology 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. A document that requires specific behavior in such an instance, rather than tolerating whatever behavior when using that facility, is non a Strictly Conforming XHTML Document.

two.2. General Terms

Attribute

An attribute is a parameter to an element declared in the DTD. An aspect's type and value range, including a possible default value, are divers in the DTD.

DTD

A DTD, or certificate type definition, is a collection of XML markup declarations that, equally a collection, defines the legal structure, elements, and attributes that are available for utilize 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 independent inside elements that are organized equally defined in the associated DTD. Run across Document Conformance for more information.

Element

An element is a document structuring unit declared in the DTD. The element's content model is divers in the DTD, and additional semantics may be defined in the prose clarification of the chemical element.

Facilities

Facilities are elements, attributes, and the semantics associated with those elements and attributes.

Implementation

See User Amanuensis.

Parsing

Parsing is the human action whereby a certificate is scanned, and the information independent within the document is filtered into the context of the elements in which the information is structured.

Rendering

Rendering is the human action whereby the data in a document is presented. This presentation is done in the form most appropriate to the environment (east.g. aurally, visually, in print).

User Amanuensis

A user agent is a system that processes XHTML documents in accord with this specification. See User Agent Conformance for more than information.

Validation

Validation is a process whereby documents are verified against the associated DTD, ensuring that the structure, use of elements, and use of attributes are consistent with the definitions in the DTD.

Well-formed

A document is well-formed when it is structured according to the rules divers in Department 2.1 of the XML one.0 Recommendation [XML].

3. Normative Definition of XHTML ane.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. Encounter Section 3.one.2 for information on using XHTML with other namespaces, for instance, to include metadata expressed in RDF inside XHTML documents.

3.1.1. Strictly Befitting 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 post-obit criteria:

1. It must suit to the constraints expressed in one of the iii DTDs found in DTDs and in Appendix B.

2. The root element of the document must exist html.

3. The root element of the certificate must contain an xmlns annunciation 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 similar:

   <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 3 DTDs found in DTDs using the respective Formal Public Identifier. The organization identifier may exist inverse 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 one.0 Transitional//EN"      "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">  <!DOCTYPE html       PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"      "http://world wide web.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 proclamation 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 adamant past a higher-level protocol. Hither is an case of an XHTML certificate. In this example, the XML declaration is included.

<?xml version="one.0" encoding="UTF-eight"?> <!DOCTYPE html       PUBLIC "-//W3C//DTD XHTML one.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>     <championship>Virtual Library</title>   </head>   <body>     <p>Moved to <a href="http://instance.org/">example.org</a>.</p>   </trunk> </html>        

3.ane.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 1.0 documents equally defined in a higher place. Work by W3C is addressing ways to specify conformance for documents involving multiple namespaces. For an example, see [XHTML+MathML].

The following case shows the way in which XHTML 1.0 could exist used in conjunction with the MathML Recommendation:

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">   <caput>     <title>A Math Example</title>   </head>   <torso>     <p>The following is MathML markup:</p>     <math xmlns="http://www.w3.org/1998/Math/MathML">       <apply> <log/>         <logbase>           <cn> three </cn>         </logbase>         <ci> x </ci>       </apply>     </math>   </body> </html>        

The following example shows the mode in which XHTML 1.0 markup could exist incorporated into another XML namespace:

<?xml version="1.0" encoding="UTF-8"?> <!-- initially, the default namespace is "books" --> <volume xmlns='urn:loc.gov:books'     xmlns:isbn='urn:ISBN:0-395-36341-half dozen' xml:lang="en" lang="en">   <championship>Cheaper by the Dozen</title>   <isbn:number>1568491379</isbn:number>   <notes>     <!-- make HTML the default namespace for a hypertext commentary -->     <p xmlns='http://world wide web.w3.org/1999/xhtml'>         This is besides available <a href="http://www.w3.org/">online</a>.     </p>   </notes> </volume>        

3.2. User Agent Conformance

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

1. In order to exist consistent with the XML 1.0 Recommendation [XML], the user agent must parse and evaluate an XHTML document for well-formedness. If the user amanuensis claims to be a validating user agent, information technology must too validate documents against their referenced DTDs according to [XML].
2. When the user amanuensis claims to support facilities divers within this specification or required by this specification through normative reference, it must do and so in ways consistent with the facilities' definition.
3. When a user agent processes an XHTML document every bit generic XML, it shall only recognize attributes of blazon ID (i.due east. the id attribute on most XHTML elements) every bit fragment identifiers.
4. If a user agent encounters an element information technology does not recognize, it must process the element'due south content.
5. If a user amanuensis encounters an attribute it does non recognize, information technology must ignore the entire attribute specification (i.e., the attribute and its value).
6. If a user agent encounters an attribute value information technology does not recognize, information technology must apply the default attribute value.
7. If it encounters an entity reference (other than i of the entities defined in this recommendation or in the XML recommendation) for which the user agent has processed no declaration (which could happen if the proclamation is in the external subset which the user agent hasn't read), the entity reference should be processed equally the characters (starting with the ampersand and catastrophe with the semi-colon) that make upward the entity reference.
8. When processing content, user agents that meet characters or character entity references that are recognized just not renderable may substitute another rendering that gives the same pregnant, or must brandish the document in such a style that it is obvious to the user that normal rendering has non taken place.

9. White space is handled co-ordinate to the following rules. The following characters are defined in [XML] white space characters:

   • Infinite ( )
   • HORIZONTAL TABULATION (	)
   • Wagon Render (
)
   • LINE FEED (
)

   The XML processor normalizes different systems' line end codes into one unmarried LINE FEED character, that is passed upwardly 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 futurity version of CSS, at which time this reference will be updated.

Notation that in order to produce a Approved XHTML certificate, the rules to a higher place must be applied and the rules in [XMLC14N] must also be applied to the document.

4. Differences with HTML iv

This section is informative.

Due to the fact that XHTML is an XML awarding, certain practices that were perfectly legal in SGML-based HTML 4 [HTML4] must be inverse.

4.1. Documents must be well-formed

Well-formedness is a new concept introduced by [XML]. Substantially 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, information technology is widely tolerated in existing browsers.

Correct: nested elements.

<p>hither is an emphasized <em>paragraph</em>.</p>

INCORRECT: overlapping elements

<p>here is an emphasized <em>paragraph.</p></em>

4.ii. Chemical element and attribute names must be in lower instance

XHTML documents must utilise lower case for all HTML chemical element and aspect names. This difference is necessary because XML is case-sensitive e.g. <li> and <LI> are dissimilar tags.

iv.3. For non-empty elements, end tags are required

In SGML-based HTML iv certain elements were permitted to omit the end tag; with the elements that followed implying closure. XML does not allow finish tags to be omitted. All elements other than those declared in the DTD as EMPTY must have an stop tag. Elements that are declared in the DTD as EMPTY tin can take an end tag or can use empty element shorthand (run into Empty Elements).

Correct: terminated elements

<p>hither is a paragraph.</p><p>here is another paragraph.</p>

Incorrect: unterminated elements

<p>here is a paragraph.<p>here is some other paragraph.

4.4. Attribute values must always be quoted

All attribute values must be quoted, even those which appear to exist numeric.

CORRECT: quoted attribute values

<td rowspan="iii">

Incorrect: unquoted attribute values

<td rowspan=3>

4.5. Aspect Minimization

XML does not back up attribute minimization. Attribute-value pairs must be written in full. Attribute names such as compact and checked cannot occur in elements without their value beingness specified.

Right: unminimized attributes

<dl compact="meaty">

INCORRECT: minimized attributes

<dl meaty>

4.6. Empty Elements

Empty elements must either take an end tag or the start tag must terminate with />. For instance, <br/> or <hr></hr>. See HTML Compatibility Guidelines for information on means to ensure this is backward compatible with HTML 4 user agents.

CORRECT: terminated empty elements

<br/><hr/>

INCORRECT: unterminated empty elements

<br><hr>

iv.7. White Space handling in attribute values

When user agents procedure attributes, they practise and then co-ordinate to Section three.3.three of [XML]:

• Strip leading and trailing white space.
• Map sequences of ane or more white space characters (including line breaks) to a single inter-discussion infinite.

4.8. Script and Fashion elements

In XHTML, the script and manner elements are declared as having #PCDATA content. As a result, < and & will exist treated equally 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 mode chemical 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 ane.3 of the DOM Level 1 Recommendation [DOM].

An culling is to use external script and manner documents.

iv.nine. SGML exclusions

SGML gives the writer of a DTD the power to exclude specific elements from being contained inside 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 some other 'a' chemical 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, sure elements should non be nested. A summary of such elements and the elements that should not be nested in them is found in the normative Chemical element Prohibitions.

4.ten. The elements with 'id' and 'name' attributes

HTML 4 divers the proper 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 in that location can only be a unmarried aspect of blazon ID per element. Therefore, in XHTML 1.0 the id attribute is divers to exist of type ID. In order to ensure that XHTML 1.0 documents are well-structured XML documents, XHTML 1.0 documents MUST utilise the id attribute when defining fragment identifiers on the elements listed to a higher place. See the HTML Compatibility Guidelines for information on ensuring such anchors are backward compatible when serving XHTML documents as media type text/html.

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

4.eleven. Attributes with pre-divers value sets

HTML four and XHTML both have some attributes that accept pre-defined and limited sets of values (eastward.yard. the blazon aspect of the input element). In SGML and XML, these are called enumerated attributes. Nether 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 estimation of these values is instance-sensitive, and in XHTML 1 all of these values are defined in lower-example.

four.12. Entity references as hex values

SGML and XML both permit references to characters past using hexadecimal values. In SGML these references could be made using either &#Xnn; or &#xnn;. In XML documents, y'all must employ the lower-case version (i.eastward. &#xnn;)

5. Compatibility Problems

This section is normative.

Although there is no requirement for XHTML one.0 documents to exist compatible with existing user agents, in practice this is easy to accomplish. Guidelines for creating compatible documents can exist found in Appendix C.

5.1. Internet Media Type

XHTML Documents which follow the guidelines set forth in Appendix C, "HTML Compatibility Guidelines" may exist labeled with the Internet Media Blazon "text/html" [RFC2854], every bit they are compatible with most HTML browsers. Those documents, and any other document befitting to this specification, may also exist labeled with the Internet Media Blazon "application/xhtml+xml" as defined in [RFC3236]. For further information on using media types with XHTML, see the informative annotation [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 announcement 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 utilise those archives rather than using the specific DTDs referenced below.

A.one. Certificate Type Definitions

These DTDs approximate the HTML 4 DTDs. The W3C recommends that you use the authoritative versions of these DTDs at their defined Organisation identifiers when validating content. If you need to employ these DTDs locally you should download one of the archives of this version. For completeness, the normative versions of the DTDs are included here:

A.1.1. XHTML-one.0-Strict

The file DTD/xhtml1-strict.dtd is a normative part of this specification. The annotated contents of this file are available in this divide department for completeness.

A.i.two. XHTML-1.0-Transitional

The file DTD/xhtml1-transitional.dtd is a normative function of this specification. The annotated contents of this file are available in this separate section for completeness.

A.1.3. XHTML-1.0-Frameset

The file DTD/xhtml1-frameset.dtd is a normative function of this specification. The annotated contents of this file are bachelor in this dissever section for abyss.

A.ii. Entity Sets

The XHTML entity sets are the aforementioned as for HTML 4, just take been modified to be valid XML ane.0 entity declarations. Notation the entity for the Euro currency sign (€ or € or €) is defined as part of the special characters.

A.two.i. Latin-1 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 abyss.

A.2.2. Special characters

The file DTD/xhtml-special.ent is a normative part of this specification. The annotated contents of this file are available in this split section for abyss.

A.2.3. Symbols

The file DTD/xhtml-symbol.ent is a normative part of this specification. The annotated contents of this file are bachelor in this split section for completeness.

B. Chemical element Prohibitions

This appendix is normative.

The following elements have prohibitions on which elements they can contain (meet SGML Exclusions). This prohibition applies to all depths of nesting, i.eastward. it contains all the descendant elements.

a

must not contain other a elements.

pre

must not contain the img, object, big, pocket-sized, sub, or sup elements.

push

must not contain the input, select, textarea, label, push button, form, fieldset, iframe or isindex elements.

label

must not contain other label elements.

grade

must non contain other form elements.

C. HTML Compatibility Guidelines

This appendix is informative.

This appendix summarizes design guidelines for authors who wish their XHTML documents to return on existing HTML user agents. Note that this recommendation does not define how HTML conforming user agents should procedure HTML documents. Nor does it ascertain the meaning of the Internet Media Type text/html. For these definitions, see [HTML4] and [RFC2854] respectively.

C.one. Processing Instructions and the XML Annunciation

Be aware that processing instructions are rendered on some user agents. Also, some user agents translate the XML announcement 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. Call up, notwithstanding, that when the XML declaration is not included in a document, the certificate tin only utilise the default character encodings UTF-eight or UTF-xvi.

C.2. Empty Elements

Include a infinite before the abaft / and > of empty elements, e.g. <br />, <60 minutes /> and <img src="karen.jpg" alt="Karen" />. Also, employ the minimized tag syntax for empty elements, e.one thousand. <br />, equally the culling syntax <br></br> allowed by XML gives uncertain results in many existing user agents.

C.three. Element Minimization and Empty Chemical element Content

Given an empty instance of an element whose content model is not EMPTY (for case, an empty championship or paragraph) exercise not employ the minimized class (e.g. utilize <p> </p> and not <p />).

C.4. Embedded Style Sheets and Scripts

Utilize external way sheets if your style sheet uses < or & or ]]> or --. Use external scripts if your script uses < or & or ]]> or --. Notation that XML parsers are permitted to silently remove the contents of comments. Therefore, the historical do of "hiding" scripts and fashion 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 Aspect Values

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

C.6. Isindex

Don't include more than one isindex element in the certificate head. The isindex element is deprecated in favor of the input chemical element.

C.7. The lang and xml:lang Attributes

Use both the lang and xml:lang attributes when specifying the language of an element. The value of the xml:lang attribute takes precedence.

C.8. Fragment Identifiers

In XML, URI-references [RFC2396] that end with fragment identifiers of the course "#foo" do not refer to elements with an attribute name="foo"; rather, they refer to elements with an attribute divers to be of type ID, e.one thousand., the id attribute in HTML four. 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.grand., <a id="foo" name="foo">...</a>).

Further, since the set of legal values for attributes of blazon ID is much smaller than for those of type CDATA, the type of the name aspect has been changed to NMTOKEN. This aspect is constrained such that it can simply have the same values as type ID, or as the Name production in XML 1.0 Section ii.iii, production v. 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 Department 2.three, product 5 is much larger than that permitted to be used in the ID and Name types divers in HTML iv. When defining fragment identifiers to be backward-compatible, only strings matching the pattern [A-Za-z][A-Za-z0-9:_.-]* should be used. Come across Section 6.2 of [HTML4] for more information.

Finally, note that XHTML 1.0 has deprecated the name aspect of the a, applet, form, frame, iframe, img, and map elements, and information technology will exist removed from XHTML in subsequent versions.

C.9. Character Encoding

Historically, the grapheme encoding of an HTML document is either specified past a spider web server via the charset parameter of the HTTP Content-Blazon header, or via a meta element in the document itself. In an XML document, the graphic symbol encoding of the document is specified on the XML annunciation (e.grand., <?xml version="one.0" encoding="EUC-JP"?>). In order to portably present documents with specific grapheme encodings, the all-time arroyo is to ensure that the web server provides the correct headers. If this is not possible, a document that wants to gear up its character encoding explicitly must include both the XML declaration an encoding declaration and a meta http-equiv argument (e.chiliad., <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: exist aware that if a certificate must include the character encoding declaration in a meta http-equiv argument, that certificate may ever be interpreted by HTTP servers and/or user agents equally being of the internet media type defined in that argument. If a document is to be served every bit multiple media types, the HTTP server must be used to gear up the encoding of the certificate.

C.ten. Boolean Attributes

Some HTML user agents are unable to interpret boolean attributes when these appear in their full (non-minimized) form, equally required by XML i.0. Note this problem doesn't bear upon user agents compliant with HTML 4. The following attributes are involved: meaty, 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 certificate object model interfaces for XML and HTML four. The HTML 4 document object model specifies that HTML element and attribute names are returned in upper-case. The XML certificate object model specifies that element and aspect names are returned in the example they are specified. In XHTML one.0, elements and attributes are specified in lower-example. This apparent difference can be addressed in two ways:

1. User agents that admission XHTML documents served as Cyberspace media type text/html via the DOM tin can use the HTML DOM, and can rely upon element and attribute names being returned in upper-case from those interfaces.
2. User agents that access XHTML documents served as Net media types text/xml, awarding/xml, or application/xhtml+xml can also use the XML DOM. Elements and attributes volition be returned in lower-case. Also, some XHTML elements may or may not appear in the object tree considering they are optional in the content model (e.grand. 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 inapplicable elements, XHTML has made the elements optional. User agents need to adjust to this accordingly. For farther information on this topic, come across [DOM2]

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

In both SGML and XML, the ampersand graphic symbol ("&") declares the offset of an entity reference (e.g., &reg; for the registered trademark symbol "®"). Unfortunately, many HTML user agents take silently ignored wrong usage of the ampersand graphic symbol in HTML documents - treating ampersands that do not look similar entity references as literal ampersands. XML-based user agents will not tolerate this incorrect usage, and any document that uses an ampersand incorrectly will non exist "valid", and consequently will not arrange to this specification. In guild 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 exist expressed themselves equally an entity reference (e.m. "&"). For example, when the href attribute of the a chemical element refers to a CGI script that takes parameters, it must be expressed equally http://my.site.dom/cgi-bin/myscript.pl?class=guest&proper noun=user rather than every bit http://my.site.dom/cgi-bin/myscript.pl?course=guest&name=user.

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

The Cascading Style Sheets level two Recommendation [CSS2] defines fashion 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:

1. CSS style sheets for XHTML should use lower case element and attribute names.
2. In tables, the tbody element will exist inferred past the parser of an HTML user agent, only not by the parser of an XML user agent. Therefore you should e'er explicitly add a tbody element if it is referred 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, fashion 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 use to XHTML documents delivered as HTML and the XML rules apply to XHTML documents delivered as XML.

C.xiv. Referencing Style Elements when serving as XML

In HTML 4 and XHTML, the style chemical element can be used to ascertain certificate-internal style rules. In XML, an XML stylesheet declaration is used to ascertain style rules. In lodge to be compatible with this convention, way elements should take their fragment identifier fix using the id attribute, and an XML stylesheet annunciation 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" blazon="text/css"?> <!DOCTYPE html       PUBLIC "-//W3C//DTD XHTML i.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"> <caput> <championship>An internal stylesheet example</title> <fashion type="text/css" id="internalStyle">   lawmaking {     colour: green;     font-family: monospace;     font-weight: bold;   } </manner> </head> <torso> <p>   This is text that uses our    <code>internal stylesheet</code>. </p> </trunk> </html>        

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

Some characters that are legal in HTML documents, are illegal in XML document. For instance, in HTML, the Formfeed character (U+000C) is treated as white space, in XHTML, due to XML'due south definition of characters, information technology is illegal.

C.sixteen. The Named Graphic symbol Reference &apos;

The named character reference ' (the apostrophe, U+0027) was introduced in XML ane.0 but does not appear in HTML. Authors should therefore use ' instead of ' to work as expected in HTML four 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 2nd edition, the membership was:

Steven Pemberton, CWI/W3C (HTML Working Grouping 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 Applied science
Ann Navarro, WebGeek, Inc.
Subramanian Peruvemba, Oracle
Rob Relyea, Microsoft
Sebastian Schnitzenbaumer, SAP
Peter Stark, Sony Ericsson

At publication of the commencement 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 10 Kate, Philips Electronics
Peter Male monarch, Phone.com
Paula Klante, JetForm
Shin'ichi Matsui, Panasonic (W3C visiting engineer through September 1999)
Shane McCarron, Practical Testing and Applied science (The Open Group through Baronial 1999)
Ann Navarro, HTML Writers Gild
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 ii (CSS2) Specification", B. Bos, H. W. Lie, C. Lilley, I. Jacobs, 12 May 1998.
Latest version bachelor at: http://www.w3.org/TR/REC-CSS2

[DOM]

"Certificate Object Model (DOM) Level ane Specification", Lauren Wood et al., 1 Oct 1998.
Latest version available at: http://www.w3.org/TR/REC-DOM-Level-one

[DOM2]

"Certificate Object Model (DOM) Level two Cadre Specification", A. Le Hors, et al., thirteen November 2000.
Latest version available at: http://world wide web.w3.org/TR/DOM-Level-two-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-i:1990 Information technology - Portable Operating System Interface (POSIX) - Part 1: Arrangement Application Program Interface (API) [C Linguistic communication]", Institute of Electrical and Electronics Engineers, Inc, 1990.

[RFC2045]

"Multipurpose Net Mail Extensions (MIME) Part One: Format of Cyberspace Message Bodies", Northward. Freed and N. Borenstein, November 1996. Note that this RFC obsoletes RFC1521, RFC1522, and RFC1590.

[RFC2046]

"RFC2046: Multipurpose Cyberspace Mail Extensions (MIME) Part 2: Media Types", North. Freed and N. Borenstein, November 1996.
Available at http://world wide web.ietf.org/rfc/rfc2046.txt. Notation that this RFC obsoletes RFC1521, RFC1522, and RFC1590.

[RFC2119]

"RFC2119: Key words for use in RFCs to Betoken Requirement Levels", S. Bradner, March 1997.
Available at: http://www.ietf.org/rfc/rfc2119.txt

[RFC2376]

"RFC2376: XML Media Types", E. Whitehead, Thou. Murata, July 1998.
This certificate is obsoleted by [RFC3023].
Available at: http://www.ietf.org/rfc/rfc2376.txt

[RFC2396]

"RFC2396: Compatible 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, Due south. St.Laurent, D. Kohn, Jan 2001.
This certificate obsoletes [RFC2376].
Available at: http://www.ietf.org/rfc/rfc3023.txt

[RFC3066]

"Tags for the Identification of Languages", H. Alvestrand, Jan 2001.
Available at: http://www.ietf.org/rfc/rfc3066.txt

[RFC3236]

"The 'application/xhtml+xml' Media Type", M. Baker, P. Stark, Jan 2002.
Available at: http://www.ietf.org/rfc/rfc3236.txt

[XHTML+MathML]

"XHTML plus Math 1.one DTD ", "A.2 MathML as a DTD Module", Mathematical Markup Language (MathML) Version 2.0. Available at: http://world wide web.w3.org/TR/MathML2/dtd/xhtml-math11-f.dtd

[XHTMLMIME]

"XHTML Media Types", Masayasu Ishikawa, ane Baronial 2002.
Latest version available at: http://world wide web.w3.org/TR/xhtml-media-types

[XHTMLMOD]

"Modularization of XHTML", Chiliad. Altheim et al., x April 2001.
Latest version bachelor at: http://www.w3.org/TR/xhtml-modularization

[XML]

"Extensible Markup Language (XML) 1.0 Specification (Second Edition)", T. Bray, J. Paoli, C. G. Sperberg-McQueen, E. Maler, half dozen Oct 2000.
Latest version bachelor at: http://world wide web.w3.org/TR/REC-xml

[XMLNS]

"Namespaces in XML", T. Bray, D. Hollander, A. Layman, 14 Jan 1999.
XML namespaces provide a unproblematic method for qualifying names used in XML documents by associating them with namespaces identified by URI.
Latest version bachelor 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 concrete representation, the canonical form, of an XML document.
Latest version bachelor at: http://www.w3.org/TR/xml-c14n

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Source: https://www.w3.org/TR/xhtml1/

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