XHTML 1.0: The Extensible HyperText Markup
Language
XHTML 1.0: The Extensible HyperText Markup
Language
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XHTML" 1.0:
The Extensible HyperText Markup Language
A Reformulation of HTML 4 in XML 1.0
W3C Recommendation 26 January 2000
This version:
http://www.w3.org/TR/2000/REC-xhtml1-20000126
(Postscript version,
PDF version,
ZIP archive, or
Gzip'd TAR archive)
Latest version:
http://www.w3.org/TR/xhtml1
Previous version:
http://www.w3.org/TR/1999/PR-xhtml1-19991210
Authors:
See acknowledgements.
Copyright
©2000 W3C® (MIT, INRIA, Keio), All Rights Reserved. W3C liability,
trademark,
document
use and software
licensing rules
apply.
Abstract
This specification defines 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 document has been reviewed by W3C Members and other
interested parties and has been endorsed by the Director
as a W3C Recommendation. It is a stable document and may be
used as reference material or cited as a normative
reference from another document. W3C's role in making the
Recommendation is to draw attention to the specification and
to promote its widespread deployment. This enhances the
functionality and interoperability of the Web.
This document has been produced as part of the W3C HTML Activity. The goals of
the HTML Working
Group (members
only) are discussed in the HTML Working Group
charter (members
only).
A list of current W3C Recommendations and other technical documents
can be found at http://www.w3.org/TR.
Public discussion on HTML features takes place on the mailing list www-html@w3.org (archive).
Please report errors in this document to www-html-editor@w3.org.
The list of known errors in this specification is available at http://www.w3.org/2000/01/REC-xhtml1-20000126-errata.
Contents
1. What is XHTML?
1.1 What is HTML 4?
1.2 What is XML?
1.3 Why the need for XHTML?
2. Definitions
2.1 Terminology
2.2 General Terms
3. Normative Definition of XHTML 1.0
3.1 Document Conformance
3.2 User Agent Conformance
4. Differences with HTML 4
5. Compatibility Issues
5.1 Internet Media Types
6. Future Directions
6.1 Modularizing HTML
6.2 Subsets and Extensibility
6.3 Document Profiles
Appendix A. DTDs
Appendix B. Element
Prohibitions
Appendix C. HTML Compatibility Guidelines
Appendix D. Acknowledgements
Appendix E. References
1. What is XHTML?
XHTML is a family of current and future document types and modules that
reproduce, subset, and extend HTML 4 [HTML]. 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 the section on Future
Directions.
XHTML 1.0 (this specification) is the first document type in the XHTML
family. It is a reformulation of the three HTML 4 document types as
applications of XML 1.0 [XML]. It is intended
to be used as a language for content that is both XML-conforming and, if some
simple guidelines are followed,
operates in HTML 4 conforming user agents. Developers who migrate
their content to XHTML 1.0 will realize the following benefits:
XHTML documents are XML conforming. As such, they are readily viewed,
edited, and validated with standard XML tools.
XHTML documents can be written to
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 [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 [HTML] 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 compatibility problems for documents across different
platforms.
As the heterogeneity of both software and platforms rapidly
proliferate, it is clear that the suitability of 'classic' HTML 4
for use on these platforms is somewhat limited.
1.2 What is XML?
XML" is the shorthand for Extensible Markup
Language, and is an acronym of Extensible Markup Language [XML].
XML was conceived as a means of regaining the power and
flexibility of SGML without most of its complexity. Although a
restricted form of SGML, XML nonetheless preserves most of SGML's
power and richness, and yet still retains all of SGML's commonly
used features.
While retaining these beneficial features, XML removes many of
the more complex features of SGML that make the authoring and
design of suitable software both difficult and costly.
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 forthcoming 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. Some estimates indicate that by the year 2002, 75% of
Internet document viewing will be carried out on these alternate
platforms. 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
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]:
Implementation-defined
A value or behavior is implementation-defined when it is left
to the implementation to define [and document] the corresponding
requirements for correct document construction.
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 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".
Reserved
A value or behavior is unspecified, but it is not allowed to
be used by Conforming Documents nor to be supported by a
Conforming User Agents.
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
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 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
Functionality includes elements,
attributes, and the semantics
associated with those elements and
attributes. An implementation
supporting that functionality is said to provide the necessary
facilities.
Implementation
An implementation is a system that provides collection of
facilities and services that supports
this specification. See User Agent
Conformance for more information.
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 an implementation
that retrieves and processes XHTML documents. See 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 Section 2.1 of
the XML 1.0 Recommendation [XML].
Basically, this definition states that elements, delimited by
their start and end tags, are nested properly within one
another.
3. Normative Definition of
XHTML 1.0
3.1 Document
Conformance
This version of XHTML provides a definition of strictly
conforming XHTML documents, which are restricted to tags and
attributes from the XHTML namespace. See 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 a document that
requires only the facilities described as mandatory in this
specification. Such a document must meet all of the following
criteria:
It must validate against one of the three DTDs found in Appendix A.
The root element of the document must be
<html>.
The root element of the document must designate the XHTML
namespace using the xmlns attribute [XMLNAMES]. The namespace for XHTML is
defined to be
http://www.w3.org/1999/xhtml.
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 Appendix A using the respective
Formal Public Identifier. The system identifier may be changed to reflect
local system conventions.
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"DTD/xhtml1-strict.dtd">
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"DTD/xhtml1-transitional.dtd">
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"DTD/xhtml1-frameset.dtd">
Here is an example of a minimal XHTML document.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<title>Virtual Library</title>
</head>
<body>
<p>Moved to <a href="http://vlib.org/">vlib.org</a>.</p>
</body>
</html>
Note that in this example, the XML declaration is included. An XML
declaration like the one above is
not required in all XML documents. 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.
3.1.2 Using XHTML with
other namespaces
The XHTML namespace may be used with other XML namespaces
as per [XMLNAMES], although such
documents are not strictly conforming XHTML 1.0 documents as
defined above. Future work by W3C will address ways to specify
conformance for documents involving multiple namespaces.
The following example shows the way in which XHTML 1.0 could
be used in conjunction with the MathML Recommendation:
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<title>A Math Example</title>
</head>
<body>
<p>The following is MathML markup:</p>
<math xmlns="http://www.w3.org/1998/Math/MathML">
<apply> <log/>
<logbase>
<cn> 3 </cn>
</logbase>
<ci> x </ci>
</apply>
</math>
</body>
</html>
The following example shows the way in which XHTML 1.0 markup
could be incorporated into another XML namespace:
<?xml version="1.0" encoding="UTF-8"?>
<!-- initially, the default namespace is "books" -->
<book xmlns='urn:loc.gov:books'
xmlns:isbn='urn:ISBN:0-395-36341-6' xml:lang="en" lang="en">
<title>Cheaper by the Dozen</title>
<isbn:number>1568491379</isbn:number>
<notes>
<!-- make HTML the default namespace for a hypertext commentary -->
<p xmlns='http://www.w3.org/1999/xhtml'>
This is also available <a href="http://www.w3.org/">online</a>.
</p>
</notes>
</book>
3.2 User Agent
Conformance
A conforming user agent must meet all of the following
criteria:
In order to be consistent with the XML 1.0 Recommendation [XML], the user agent must parse and evaluate
an XHTML document for well-formedness. If the user agent claims
to be a validating user agent, it must also validate documents
against their referenced DTDs according to
[XML].
When the user agent claims to support
facilities defined within this specification or required by
this specification through normative reference, it must do so in
ways consistent with the facilities' definition.
When a user agent processes an XHTML document as generic XML,
it shall only recognize attributes of type
ID (e.g. the id attribute on most XHTML elements)
as fragment identifiers.
If a user agent encounters an element it does not recognize,
it must render the element's content.
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).
If a user agent encounters an attribute value it doesn't
recognize, it must use the default attribute value.
If it encounters an entity reference (other than one
of the predefined entities) 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 rendered as the characters (starting
with the ampersand and ending with the semi-colon) that
make up the entity reference.
When rendering content, User Agents that encounter
characters or character entity references that are recognized but not renderable should display the document in such a way that it is obvious to the user that normal rendering has not taken place.
The following characters are defined in [XML] as whitespace characters:
Space ( )
Tab (	)
Carriage return (
)
Line feed (
)
The XML processor normalizes different system's line end codes into one
single line-feed character, that is passed up to the application. The XHTML
user agent in addition, must treat the following characters as whitespace:
Form feed ()
Zero-width space (​)
In elements where the 'xml:space' attribute is set to 'preserve', the user
agent must leave all whitespace characters intact (with the exception of
leading and trailing whitespace characters, which should be removed).
Otherwise, whitespace
is handled according to the following rules:
All whitespace surrounding block elements should be removed.
Comments are removed entirely and do not affect whitespace handling. One
whitespace character on either side of a comment is treated as two white
space characters.
Leading and trailing whitespace inside a block element must be removed.
Line feed characters within a block element must be converted into a
space (except when the 'xml:space' attribute is set to 'preserve').
A sequence of white space characters must be reduced to a single space
character (except when the 'xml:space' attribute is set to 'preserve').
With regard to rendition,
the User Agent should render the content in a
manner appropriate to the language in which the content is written.
In languages whose primary script is Latinate, the ASCII space
character is typically used to encode both grammatical word boundaries and
typographic whitespace; in languages whose script is related to Nagari
(e.g., Sanskrit, Thai, etc.), grammatical boundaries may be encoded using
the ZW 'space' character, but will not typically be represented by
typographic whitespace in rendered output; languages using Arabiform scripts
may encode typographic whitespace using a space character, but may also use
the ZW space character to delimit 'internal' grammatical boundaries (what
look like words in Arabic to an English eye frequently encode several words,
e.g. 'kitAbuhum' = 'kitAbu-hum' = 'book them' == their book); and languages
in the Chinese script tradition typically neither encode such delimiters nor
use typographic whitespace in this way.
Whitespace in attribute values is processed according to [XML].
4. Differences with HTML 4
Due to the fact that XHTML is an XML application, certain
practices that were perfectly legal in SGML-based HTML 4 [HTML] must be changed.
4.1 Documents must be
well-formed
Well-formedness is a new concept
introduced by [XML]. Essentially this
means that all elements must either have closing tags or be
written in a special form (as described below), and that all the
elements must nest.
Although overlapping is illegal in SGML, it was widely
tolerated in existing browsers.
CORRECT: nested elements.
<p>here is an emphasized
<em>paragraph</em>.</p>
INCORRECT: overlapping elements
<p>here is an emphasized
<em>paragraph.</p></em>
4.2 Element and attribute
names must be in lower case
XHTML documents must use lower case for all HTML element and
attribute names. This difference is necessary because XML is
case-sensitive e.g. <li> and <LI> are different
tags.
4.3 For non-empty elements,
end tags are required
In SGML-based HTML 4 certain elements were permitted to omit
the end tag; with the elements that followed implying closure.
This omission is not permitted in XML-based XHTML. All elements
other than those declared in the DTD as EMPTY must
have an end tag.
CORRECT: terminated elements
<p>here is a paragraph.</p><p>here is
another paragraph.</p>
INCORRECT: unterminated elements
<p>here is a paragraph.<p>here is another
paragraph.
4.4 Attribute values must
always be quoted
All attribute values must be quoted, even those which appear
to be numeric.
CORRECT: quoted attribute values
<table rows="3">
INCORRECT: unquoted attribute values
<table rows=3>
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
<dl compact="compact">
INCORRECT: minimized attributes
<dl compact>
4.6 Empty Elements
Empty elements must either have an end tag or the start tag must end with />. For instance,
<br/> or <hr></hr>. See HTML Compatibility Guidelines for information on ways to
ensure this is backward compatible with HTML 4 user agents.
CORRECT: terminated empty tags
<br/><hr/>
INCORRECT: unterminated empty tags
<br><hr>
4.7 Whitespace handling in
attribute values
In attribute values, user agents will strip leading and
trailing whitespace from attribute values and map sequences
of one or more whitespace characters (including line breaks) to
a single inter-word space (an ASCII space character for western
scripts). See
Section 3.3.3 of [XML].
4.8 Script and Style
elements
In XHTML, the script and style elements are declared as having
#PCDATA content. As a result, < and
& will be treated as the start of markup, and
entities such as < and &
will be recognized as entity references by the XML processor to
< and & respectively. Wrapping
the content of the script or style element within a
CDATA marked section avoids the expansion of these
entities.
<script>
<![CDATA[
... unescaped script content ...
]]>
</script>
CDATA sections are recognized by the XML
processor and appear as nodes in the Document Object Model, see
Section 1.3 of the DOM Level 1 Recommendation [DOM].
An alternative is to use external script and style
documents.
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
Appendix B.
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, even on elements that historically have also had a
name attribute.
See the HTML Compatibility
Guidelines for information on ensuring such anchors are backwards
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.
5. Compatibility Issues
Although there is no requirement for XHTML 1.0 documents to be
compatible with existing user agents, in practice this is easy to
accomplish. Guidelines for creating compatible documents can be
found in Appendix C.
5.1 Internet Media Type
As of the publication of this recommendation, the general
recommended MIME labeling for XML-based applications
has yet to be resolved.
However, XHTML Documents which follow the guidelines set forth
in Appendix C, "HTML Compatibility Guidelines" may be
labeled with the Internet Media Type "text/html", as they
are compatible with most HTML browsers. This document
makes no recommendation about MIME labeling of other
XHTML documents.
6. Future Directions
XHTML 1.0 provides the basis for a family of document types
that will extend and subset XHTML, in order to support a wide
range of new devices and applications, by defining modules and
specifying a mechanism for combining these modules. This
mechanism will enable the extension and sub-setting of XHTML 1.0
in a uniform way through the definition of new modules.
6.1 Modularizing HTML
As the use of XHTML moves from the traditional desktop user
agents to other platforms, it is clear that not all of the XHTML
elements will be required on all platforms. For example a hand
held device or a cell-phone may only support a subset of XHTML
elements.
The process of modularization breaks XHTML up into a series of
smaller element sets. These elements can then be recombined to
meet the needs of different communities.
These modules will be defined in a later W3C document.
6.2 Subsets and
Extensibility
Modularization brings with it several advantages:
It provides a formal mechanism for sub-setting XHTML.
It provides a formal mechanism for extending XHTML.
It simplifies the transformation between document types.
It promotes the reuse of modules in new document types.
6.3 Document
Profiles
A document profile specifies the syntax and semantics of a set
of documents. Conformance to a document profile provides a basis
for interoperability guarantees. The document profile specifies
the facilities required to process documents of that type, e.g.
which image formats can be used, levels of scripting, style sheet
support, and so on.
For product designers this enables various groups to define
their own standard profile.
For authors this will obviate the need to write several
different versions of documents for different clients.
For special groups such as chemists, medical doctors, or
mathematicians this allows a special profile to be built using
standard HTML elements plus a group of elements geared to the
specialist's needs.
Appendix 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 zip file for this specification.
A.1 Document Type
Definitions
These DTDs approximate the HTML 4 DTDs. It is likely that
when the DTDs are modularized, a method of DTD construction will
be employed that corresponds more closely to HTML 4.
XHTML-1.0-Strict
XHTML-1.0-Transitional
XHTML-1.0-Frameset
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.
Latin-1 characters
Special characters
Symbols
Appendix B. Element
Prohibitions
This appendix is normative.
The following elements have prohibitions on which elements
they can contain (see Section 4.9). This
prohibition applies to all depths of nesting, i.e. it contains
all the descendant elements.
a
cannot contain other a elements.
pre
cannot contain the img, object,
big, small, sub, or
sup elements.
button
cannot contain the input, select,
textarea, label, button,
form, fieldset, iframe or
isindex elements.
label
cannot contain other label elements.
form
cannot contain other form elements.
Appendix 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.
C.1 Processing Instructions
Be aware that processing instructions are rendered on some
user agents. However, also note that when the XML declaration is not included
in a document, the document can only use the default character encodings UTF-8
or UTF-16.
C.2 Empty Elements
Include a space before the trailing / and
> of empty elements, e.g.
<br />,
<hr /> and <img
src="karen.jpg" alt="Karen" />. Also, use the
minimized tag syntax for empty elements, e.g. <br />, as the alternative syntax <br></br> allowed by XML
gives uncertain results in many existing user agents.
C.3 Element Minimization and Empty Element Content
Given an empty instance of an element whose content model is
not EMPTY (for example, an empty title or paragraph)
do not use the minimized form (e.g. use
<p> </p> and not
<p />).
C.4 Embedded Style Sheets and Scripts
Use external style sheets if your style sheet uses
< or & or ]]> or --. Use
external scripts if your script uses < or
& or ]]> or --. Note that XML parsers
are permitted to silently remove the contents of comments. Therefore, the historical
practice of "hiding" scripts and style sheets within comments to make the
documents backward compatible is likely to not work as expected in XML-based
implementations.
C.5 Line Breaks within Attribute Values
Avoid line breaks and multiple whitespace 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, URIs [RFC2396] that end with fragment identifiers of the form
"#foo" do not refer to elements with an attribute
name="foo"; rather, they refer to elements with an
attribute defined to be of type ID, e.g., the
id attribute in HTML 4. Many existing HTML clients don't
support the use of ID-type attributes in this way,
so identical values may be supplied for both of these attributes to ensure
maximum forward and backward compatibility (e.g., <a id="foo" name="foo">...</a>).
Further, since the set of
legal values for attributes of type ID is much smaller than
for those of type CDATA, the type of the name
attribute has been changed to NMTOKEN. This attribute is
constrained such that it can only have the same values as type
ID, or as the Name production in XML 1.0 Section
2.5, 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.
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
To specify a character encoding in the document, use both the
encoding attribute specification on the xml declaration (e.g.
<?xml version="1.0"
encoding="EUC-JP"?>) and a meta http-equiv statement
(e.g. <meta http-equiv="Content-type"
content='text/html; charset="EUC-JP"' />). The
value of the encoding attribute of the xml processing instruction
takes precedence.
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 [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:
Applications 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.
Applications that access XHTML documents served as Internet media types
text/xml or application/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.
Applications need to adapt to this
accordingly.
C.12 Using Ampersands in Attribute Values
When an attribute value contains an ampersand, it must be expressed as a character
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 [CSS2] defines style
properties which are applied to the parse tree of the HTML or XML
document. 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:
CSS style sheets for XHTML should use lower case element and
attribute names.
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.
Within the XHTML name space, 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.
Within the XHTML name space, user agents are expected to
recognize the "class" attribute. Therefore, style sheets should be
able to continue using the shorthand "." selector syntax.
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.
Appendix D.
Acknowledgements
This appendix is informative.
This specification was written with the participation of the
members of the W3C HTML working group:
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 (W3C lead for HTML)
Patrick Schmitz, Microsoft
Sebastian Schnitzenbaumer, Stack Overflow
Peter Stark, Phone.com
Chris Wilson, Microsoft
Ted Wugofski, Gateway 2000
Dan Zigmond, WebTV Networks
Appendix E. References
This appendix is informative.
[CSS2]
"Cascading Style Sheets, level 2 (CSS2) Specification", B.
Bos, H. W. Lie, C. Lilley, I. Jacobs, 12 May 1998.
Latest version available at:
http://www.w3.org/TR/REC-CSS2
[DOM]
"Document Object Model (DOM) Level 1 Specification", Lauren
Wood et al., 1 October 1998.
Latest version available at:
http://www.w3.org/TR/REC-DOM-Level-1
[HTML]
"HTML 4.01 Specification", D. Raggett, A. Le Hors, I.
Jacobs, 24 December 1999.
Latest version available at:
http://www.w3.org/TR/html401
[POSIX.1]
"ISO/IEC 9945-1:1990 Information Technology - Portable
Operating System Interface (POSIX) - Part 1: System Application
Program Interface (API) [C Language]", Institute of Electrical
and Electronics Engineers, Inc, 1990.
[RFC2046]
"RFC2046: Multipurpose Internet Mail Extensions (MIME) Part
Two: Media Types", N. Freed and N. Borenstein, November
1996.
Available at
http://www.ietf.org/rfc/rfc2046.txt. Note that this RFC
obsoletes RFC1521, RFC1522, and RFC1590.
[RFC2119]
"RFC2119: Key words for use in RFCs to Indicate Requirement
Levels", S. Bradner, March 1997.
Available at:
http://www.ietf.org/rfc/rfc2119.txt
[RFC2376]
"RFC2376: XML Media Types", E. Whitehead, M. Murata, July
1998.
Available at:
http://www.ietf.org/rfc/rfc2376.txt
[RFC2396]
"RFC2396: Uniform Resource Identifiers (URI): Generic
Syntax", T. Berners-Lee, R. Fielding, L. Masinter, August
1998.
This document updates RFC1738 and RFC1808.
Available at:
http://www.ietf.org/rfc/rfc2396.txt
[XML]
"Extensible Markup Language (XML) 1.0 Specification", T.
Bray, J. Paoli, C. M. Sperberg-McQueen, 10 February 1998.
Latest version available at:
http://www.w3.org/TR/REC-xml
[XMLNAMES]
"Namespaces in XML", T. Bray, D. Hollander, A. Layman, 14
January 1999.
XML namespaces provide a simple method for qualifying names used
in XML documents by associating them with namespaces identified
by URI.
Latest version available at:
http://www.w3.org/TR/REC-xml-names
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