Ergebnis für URL: http://www.w3.org/TR/xhtml1 [1]W3C
XHTML(TM) 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superseded 27 March 2018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:
[2]http://www.w3.org/TR/2018/SPSD-xhtml1-20180327/
Latest version:
[3]http://www.w3.org/TR/xhtml1
Previous version:
[4]http://www.w3.org/TR/2002/REC-xhtml1-20020801
Authors:
See [5]acknowledgments.
Please refer to the [6]errata for this document, which may include some normative
corrections. See also [7]translations.
This document is also available in these non-normative formats: [8]Multi-part
XHTML file, [9]PostScript version, [10]PDF version, [11]ZIP archive, and
[12]Gzip'd TAR archive.
[13]Copyright ©2002 [14]W3C^® ([15]MIT, [16]INRIA, [17]Keio), All Rights
Reserved. W3C [18]liability, [19]trademark, [20]document use and [21]software
licensing rules apply.
____________________________________________________________________________
Abstract
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.
Status of this document
This section describes the status of this document at the time of its
publication. Other documents may supersede this document. The latest status of
this document series is maintained at the W3C.
This specification is a [22]Superseded Recommendation. A newer specification
exists that is recommended for new adoption in place of this specification. New
implementations should follow the [23]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 August 2002. Changes between this version and the
previous Recommendation are illustrated in a [24]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.
This document has been produced as part of the [25]W3C 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 be found on the Working Group's [26]patent
disclosure page.
A list of current W3C Recommendations and other technical documents can be found
at [27]https://www.w3.org/TR/.
Quick Table of Contents
* 1. [28]What is XHTML?
* 2. [29]Definitions
* 3. [30]Normative Definition of XHTML 1.0
* 4. [31]Differences with HTML 4
* 5. [32]Compatibility Issues
* A. [33]DTDs
* B. [34]Element Prohibitions
* C. [35]HTML Compatibility Guidelines
* D. [36]Acknowledgements
* E. [37]References
Full Table of Contents
* 1. [38]What is XHTML?
+ 1.1. [39]What is HTML 4?
+ 1.2. [40]What is XML?
+ 1.3. [41]Why the need for XHTML?
* 2. [42]Definitions
+ 2.1. [43]Terminology
+ 2.2. [44]General Terms
* 3. [45]Normative Definition of XHTML 1.0
+ 3.1. [46]Document Conformance
o 3.1.1. [47]Strictly Conforming Documents
o 3.1.2. [48]Using XHTML with other namespaces
+ 3.2. [49]User Agent Conformance
* 4. [50]Differences with HTML 4
+ 4.1. [51]Documents must be well-formed
+ 4.2. [52]Element and attribute names must be in lower case
+ 4.3. [53]For non-empty elements, end tags are required
+ 4.4. [54]Attribute values must always be quoted
+ 4.5. [55]Attribute Minimization
+ 4.6. [56]Empty Elements
+ 4.7. [57]White Space handling in attribute values
+ 4.8. [58]Script and Style elements
+ 4.9. [59]SGML exclusions
+ 4.10. [60]The elements with 'id' and 'name' attributes
+ 4.11. [61]Attributes with pre-defined value sets
+ 4.12. [62]Entity references as hex values
* 5. [63]Compatibility Issues
+ 5.1. [64]Internet Media Type
* A. [65]DTDs
+ A.1. [66]Document Type Definitions
o A.1.1. [67]XHTML-1.0-Strict
o A.1.2. [68]XHTML-1.0-Transitional
o A.1.3. [69]XHTML-1.0-Frameset
+ A.2. [70]Entity Sets
o A.2.1. [71]Latin-1 characters
o A.2.2. [72]Special characters
o A.2.3. [73]Symbols
* B. [74]Element Prohibitions
* C. [75]HTML Compatibility Guidelines
+ C.1. [76]Processing Instructions and the XML Declaration
+ C.2. [77]Empty Elements
+ C.3. [78]Element Minimization and Empty Element Content
+ C.4. [79]Embedded Style Sheets and Scripts
+ C.5. [80]Line Breaks within Attribute Values
+ C.6. [81]Isindex
+ C.7. [82]The lang and xml:lang Attributes
+ C.8. [83]Fragment Identifiers
+ C.9. [84]Character Encoding
+ C.10. [85]Boolean Attributes
+ C.11. [86]Document Object Model and XHTML
+ C.12. [87]Using Ampersands in Attribute Values (and Elsewhere)
+ C.13. [88]Cascading Style Sheets (CSS) and XHTML
+ C.14. [89]Referencing Style Elements when serving as XML
+ C.15. [90]White Space Characters in HTML vs. XML
+ C.16. [91]The Named Character Reference '
* D. [92]Acknowledgements
* E. [93]References
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 [[94]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 [[95]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
[[96]XML]. It is intended to be used as a language for content that is both
XML-conforming and, if some simple [97]guidelines are followed, operates in
HTML 4 conforming user agents. Developers who migrate their content to XHTML 1.0
will realize the following benefits:
* XHTML documents are XML conforming. As such, they are readily viewed, edited,
and validated with standard XML tools.
* XHTML documents can be written to operate as well or better than they did
before in existing HTML 4-conforming user agents as well as in new, XHTML 1.0
conforming user agents.
* XHTML documents can utilize applications (e.g. scripts and applets) that rely
upon either the HTML Document Object Model or the XML Document Object Model
[[98]DOM].
* As the XHTML family evolves, documents conforming to XHTML 1.0 will be more
likely to interoperate within and among various XHTML environments.
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.
1.1. What is HTML 4?
HTML 4 [[99]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.
1.2. What is XML?
XML(TM) is the shorthand name for Extensible Markup Language [[100]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.
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:
* Document developers and user agent designers are constantly discovering new
ways to express their ideas through new markup. In XML, it is relatively easy
to introduce new elements or additional 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 mechanism, servers, proxies,
and user agents will be able to perform best effort content transformation.
Ultimately, it will be possible to develop XHTML-conforming content that is
usable by any XHTML-conforming user agent.
2. Definitions
This section is normative.
2.1. Terminology
The following terms are used in this specification. These terms extend the
definitions in [[101]RFC2119] in ways based upon similar definitions in ISO/IEC
9945-1:1990 [[102]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 to [103]Document 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 be interpreted 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. A document 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,
and attributes 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 within elements that are organized as defined in the associated
DTD. See [104]Document Conformance for more information.
Element
An element is a document structuring unit declared in the DTD. The
element's content model is defined in the DTD, and additional semantics
may be defined in the prose description of the element.
Facilities
Facilities are elements, attributes, and the semantics associated with
those elements and attributes.
Implementation
See User Agent.
Parsing
Parsing is the act whereby a document is scanned, and the information
contained within the document is filtered into the context of the elements
in which the information is structured.
Rendering
Rendering is the act whereby the information in a document 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. See [105]User Agent Conformance for more 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
defined in [106]Section 2.1 of the XML 1.0 Recommendation [[107]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. See [108]Section 3.1.2 for information on using XHTML with other
namespaces, for instance, to include metadata expressed in RDF within XHTML
documents.
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
in [109]DTDs and in [110]Appendix B.
2. The root element of the document must be html.
3. The root element of the document must contain an xmlns declaration for the
XHTML namespace [[111]XMLNS]. The namespace for XHTML is defined to be
http://www.w3.org/1999/xhtml. An example root element might look like:
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 in [112]DTDs using the respective
Formal Public Identifier. The system identifier may be changed to reflect
local system conventions.
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 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.
Virtual Library
Moved to example.org.
3.1.2. Using XHTML with other namespaces
The XHTML namespace may be used with other XML namespaces as per [[113]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 [[114]XHTML+MathML].
The following example shows the way in which XHTML 1.0 could be used in
conjunction with the MathML Recommendation:
A Math Example
The following is MathML markup:
3
x
The following example shows the way in which XHTML 1.0 markup could be
incorporated into another XML namespace:
Cheaper by the Dozen
1568491379
This is also available online.
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 [[115]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 [[116]XML].
2. When the user agent claims to support [117]facilities defined within this
specification or required by this specification through normative reference,
it must do 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 type ID (i.e. the id attribute on most XHTML
elements) as fragment identifiers.
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 (which could 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
with the 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 [[118]XML] white space characters:
+ SPACE ( )
+ HORIZONTAL TABULATION (	)
+ CARRIAGE RETURN (
)
+ LINE FEED (
)
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 [[119]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 [[120]XMLC14N] must also be applied to the 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 [[121]HTML4] must be changed.
4.1. Documents must be well-formed
[122]Well-formedness is a new concept introduced by [[123]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.
here is an emphasized paragraph.
INCORRECT: overlapping elements
here is an emphasized paragraph.
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. and 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 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 [124]Empty Elements).
CORRECT: terminated elements
here is a paragraph.here is another paragraph.
INCORRECT: unterminated elements
here is a paragraph.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
INCORRECT: unquoted attribute values
4.5. Attribute Minimization
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
INCORRECT: minimized attributes
4.6. Empty Elements
Empty elements must either have an end tag or the start tag must end with />. For
instance, or . See [125]HTML Compatibility Guidelines for
information on ways to ensure this is backward compatible with HTML 4 user
agents.
CORRECT: terminated empty elements
INCORRECT: unterminated empty elements
4.7. White Space handling in attribute values
When user agents process attributes, they do so according to [126]Section 3.3.3
of [[127]XML]:
* Strip leading and trailing white space.
* Map sequences of one or more white space characters (including line breaks)
to a single inter-word space.
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, 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.
CDATA sections are recognized by the XML processor and appear as nodes in the
Document Object Model, see [128]Section 1.3 of the DOM Level 1 Recommendation
[[129]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 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 [130]Element
Prohibitions.
4.10. The elements with 'id' and 'name' attributes
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 [131]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.
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. 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.
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 the lower-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 be found in [132]Appendix C.
5.1. Internet Media Type
XHTML Documents which follow the guidelines set forth in [133]Appendix C, "HTML
Compatibility Guidelines" may be labeled with the Internet Media Type "text/html"
[[134]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 [[135]RFC3236]. For
further information on using media types with XHTML, see the informative note
[[136]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 the [137]zip file and the [138]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.
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 use these DTDs locally you should download one
of the archives of [139]this version. For completeness, the normative versions of
the DTDs are included here:
A.1.1. XHTML-1.0-Strict
The file [140]DTD/xhtml1-strict.dtd is a normative part of this specification.
The annotated contents of this file are available in this [141]separate section
for completeness.
A.1.2. XHTML-1.0-Transitional
The file [142]DTD/xhtml1-transitional.dtd is a normative part of this
specification. The annotated contents of this file are available in this
[143]separate section for completeness.
A.1.3. XHTML-1.0-Frameset
The file [144]DTD/xhtml1-frameset.dtd is a normative part of this specification.
The annotated contents of this file are available in this [145]separate 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
(€ or € or €) is defined as part of the special characters.
A.2.1. Latin-1 characters
The file [146]DTD/xhtml-lat1.ent is a normative part of this specification. The
annotated contents of this file are available in this [147]separate section for
completeness.
A.2.2. Special characters
The file [148]DTD/xhtml-special.ent is a normative part of this specification.
The annotated contents of this file are available in this [149]separate section
for completeness.
A.2.3. Symbols
The file [150]DTD/xhtml-symbol.ent is a normative part of this specification. The
annotated contents of this file are available in this [151]separate section for
completeness.
B. Element Prohibitions
This appendix is normative.
The following elements have prohibitions on which elements they can contain (see
[152]SGML Exclusions). This prohibition applies to all depths of nesting, i.e. it
contains all the descendant elements.
a
must not contain other a elements.
pre
must not contain the img, object, big, small, sub, or sup elements.
button
must not contain the input, select, textarea, label, button, form,
fieldset, iframe or isindex elements.
label
must not contain other label elements.
form
must not 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 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 [[153]HTML4] and [[154]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, 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.
C.2. Empty Elements
Include a space before the trailing / and > of empty elements, e.g. ,
and . Also, use the minimized tag
syntax for empty elements, e.g. , as the alternative syntax
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 not EMPTY (for
example, an empty title or paragraph) do not use the minimized form (e.g. use
and not ).
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" scripts and 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 one isindex element in the document head. The isindex
element is deprecated in favor of the input 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 [[155]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., ...).
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
[156]Section 6.2 of [[157]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.
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 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., ). 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., ). 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.
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 agents compliant 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 [[158]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:
1. User agents that access XHTML documents served as Internet media type
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.
2. User agents that access XHTML documents served as Internet media types
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 [[159]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., ® 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.
C.13. Cascading Style Sheets (CSS) and XHTML
The Cascading Style Sheets level 2 Recommendation [[160]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:
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 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, 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 as XML.
C.14. Referencing Style Elements when serving as XML
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:
An internal stylesheet example
code {
color: green;
font-family: monospace;
font-weight: bold;
}
This is text that uses our
internal stylesheet.
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 of characters, it is illegal.
C.16. The Named Character Reference '
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.
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]
"[161]Cascading Style Sheets, level 2 (CSS2) Specification", B. Bos, H. W.
Lie, C. Lilley, I. Jacobs, 12 May 1998.
[162]Latest version available at: http://www.w3.org/TR/REC-CSS2
[DOM]
"[163]Document Object Model (DOM) Level 1 Specification", Lauren Wood et
al., 1 October 1998.
[164]Latest version available at: http://www.w3.org/TR/REC-DOM-Level-1
[DOM2]
"[165]Document Object Model (DOM) Level 2 Core Specification", A. Le Hors,
et al., 13 November 2000.
[166]Latest version available at: http://www.w3.org/TR/DOM-Level-2-Core
[HTML]
"[167]HTML 4.01 Specification", D. Raggett, A. Le Hors, I. Jacobs, 24
December 1999.
[168]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 Electrical and Electronics Engineers, Inc, 1990.
[RFC2045]
"[169]Multipurpose Internet Mail Extensions (MIME) Part One: Format of
Internet Message Bodies", N. Freed and N. Borenstein, November 1996. Note
that this RFC obsoletes RFC1521, RFC1522, and RFC1590.
[RFC2046]
"[170]RFC2046: Multipurpose Internet Mail Extensions (MIME) Part Two:
Media Types", N. Freed and N. Borenstein, November 1996.
Available at [171]http://www.ietf.org/rfc/rfc2046.txt. Note that this RFC
obsoletes RFC1521, RFC1522, and RFC1590.
[RFC2119]
"[172]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]
"[173]RFC2376: XML Media Types", E. Whitehead, M. Murata, July 1998.
This document is obsoleted by [[174]RFC3023].
Available at: http://www.ietf.org/rfc/rfc2376.txt
[RFC2396]
"[175]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]
"[176]RFC2854: The text/html Media Type", D. Conolly, L. Masinter, June
2000.
Available at: http://www.ietf.org/rfc/rfc2854.txt
[RFC3023]
"[177]RFC3023: XML Media Types", M. Murata, S. St.Laurent, D. Kohn,
January 2001.
This document obsoletes [[178]RFC2376].
Available at: http://www.ietf.org/rfc/rfc3023.txt
[RFC3066]
"[179]Tags for the Identification of Languages", H. Alvestrand, January
2001.
Available at: http://www.ietf.org/rfc/rfc3066.txt
[RFC3236]
"[180]The 'application/xhtml+xml' Media Type", M. Baker, P. Stark, January
2002.
Available at: http://www.ietf.org/rfc/rfc3236.txt
[XHTML+MathML]
"[181]XHTML plus Math 1.1 DTD", "A.2 MathML as a DTD Module", Mathematical
Markup Language (MathML) Version 2.0. Available at:
http://www.w3.org/TR/MathML2/dtd/xhtml-math11-f.dtd
[XHTMLMIME]
"[182]XHTML Media Types", Masayasu Ishikawa, 1 August 2002.
[183]Latest version available at: http://www.w3.org/TR/xhtml-media-types
[XHTMLMOD]
"[184]Modularization of XHTML", M. Altheim et al., 10 April 2001.
[185]Latest version available at:
http://www.w3.org/TR/xhtml-modularization
[XML]
"[186]Extensible Markup Language (XML) 1.0 Specification (Second
Edition)", T. Bray, J. Paoli, C. M. Sperberg-McQueen, E. Maler, 6 October
2000.
[187]Latest version available at: http://www.w3.org/TR/REC-xml
[XMLNS]
"[188]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.
[189]Latest version available at: http://www.w3.org/TR/REC-xml-names
[XMLC14N]
"[190]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.
[191]Latest version available at: http://www.w3.org/TR/xml-c14n
[192]Level Triple-A conformance icon, W3C-WAI Web Content Accessibility
Guidelines 1.0
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178. https://www.w3.org/TR/xhtml1/#ref-rfc2376
179. http://www.ietf.org/rfc/rfc3066.txt
180. http://www.ietf.org/rfc/rfc3236.txt
181. https://www.w3.org/TR/MathML2/dtd/xhtml-math11-f.dtd
182. https://www.w3.org/TR/2002/NOTE-xhtml-media-types-20020801
183. https://www.w3.org/TR/xhtml-media-types
184. https://www.w3.org/TR/2001/REC-xhtml-modularization-20010410
185. https://www.w3.org/TR/xhtml-modularization
186. https://www.w3.org/TR/2000/REC-xml-20001006
187. https://www.w3.org/TR/REC-xml
188. https://www.w3.org/TR/1999/REC-xml-names-19990114
189. https://www.w3.org/TR/REC-xml-names
190. https://www.w3.org/TR/2001/REC-xml-c14n-20010315
191. https://www.w3.org/TR/xml-c14n
192. https://www.w3.org/WAI/WCAG1AAA-Conformance
Usage: http://www.kk-software.de/kklynxview/get/URL
e.g. http://www.kk-software.de/kklynxview/get/http://www.kk-software.de
Errormessages are in German, sorry ;-)