WD-html-in-xml-19990224
W3C Working Draft 24th February 1999
This specification defines XHTML 1.0, a reformulation of HTML 4.0 as an XML 1.0 application, and three namespaces corresponding to the ones defined by HTML 4.0. The semantics of the elements and their attributes are defined in the W3C Recommendation for HTML 4.0. These semantics provide the foundation for future extensibility of XHTML. Compatibility with existing HTML user agents is possible by following a small set of guidelines.
This working draft may be updated, replaced or rendered obsolete by other W3C documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress". This document is work in progress and does not imply endorsement by the W3C membership.
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).
Please send detailed comments on this document to [email protected]. We cannot guarantee a personal response, but we will try when it is appropriate. Public discussion on HTML features takes place on the mailing list [email protected]. The W3C staff contact for work on HTML is Dave Raggett.
Appendices
Appendix A. DTDs
Appendix B. Element Prohibitions
Appendix C. Guidelines
Appendix D. References
XHTML is a reformulation of HTML 4.0 [HTML] as an application of XML 1.0 [XML].
XHTML 1.0 specifies three XML namespaces, corresponding to the three HTML 4.0 DTDs: Strict, Transitional, and Frameset. Each of these three namespaces is identified by its own URI.
XHTML 1.0 is the basis for a family of future document types that extend and subset HTML. This idea is discussed in more detail in the section on Future Directions.
HTML 4.0 [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 4.0 for use on these platforms is somewhat limited.
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.
There are two major reasons for content developers to adopt XHTML:
First, XHTML is designed to be extensible. This extensibility relies upon the XML requirement that documents be well-formed. Under SGML, the addition of a new group of elements would mean alteration of the entire DTD. In an XML-based DTD, all that is required is that the new set of elements be internally consistent and well-formed to be added to an existing DTD. The greatly eases the development and integration of new collections of elements.
Second, XHTML is designed for portability. There will be increasing use of non-desktop user agents to access Internet documents. Some estimates indicate that by the year 2002, 75% of Internet document viewing will be carried out on these alternate platforms. In most cases these platforms will not have the computing power of a desktop platform, and will not be designed to accommodate ill-formed HTML as current user agents tend to do. Indeed if these user agents do not receive well-formed XHTML, they may simply not display the document.
The following terms are used in this specification. These terms extend the definitions in [RFC2119] in ways based upon similar definitions in ISO/IEC 9945-1:1990 [POSIX.1]:
This version of XHTML only defines document conformance in terms of a Strictly Conforming XHTML Document. 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 one of three
defined namespaces by using the xmlns attribute
[XMLNAMES]. The namespace designated must
match that of the DTD that the document purports to validate against.
The defined namespaces are:
http://www.w3.org/Profiles/xhtml1-strict.dtd
http://www.w3.org/Profiles/xhtml1-transitional.dtd
http://www.w3.org/Profiles/xhtml1-frameset.dtd
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 modified appropriately.
<!DOCTYPE
html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"xhtml1-strict.dtd">
<!DOCTYPE
html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"xhtml1-transitional.dtd">
<!DOCTYPE
html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"xhtml1-frameset.dtd">
A Strictly Conforming XHTML Document may be labeled with the
Internet Media Type text/html or
text/xhtml. When labeled as text/html,
documents should follow the guidelines set forth in Appendix C. Failure to follow these
guidelines will almost certainly ensure that the document will fail
to be processed on older implementations.
Here is an example of a minimal XHTML document.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/Profiles/xhtml1-strict.dtd">
<head>
<title>Virtual Library</title>
</head>
<body>
<p>Moved to <a href="http://www.vlib.org/">www.vlib.org</a>.</p>
</body>
</html>
A conforming user agent must meet all of the following criteria:
Due to the fact that XHTML is an XML application, certain practices that were perfectly legal in SGML-based HTML 4.0 [HTML] must be changed.
Well-formedness is a new concept introduced by [XML]. Essentially this means that all elements must either have closing tags or be written in a special form (as described below), and that all the elements must nest.
Although overlapping is illegal in SGML, it was widely tolerated in SGML-based browsers.
CORRECT: nested elements.
<p>here is an emphasized <em>paragraph</em>.</p>
INCORRECT: overlapping elements
<p>here is an emphasized <em>paragraph.</p></em>
XHTML documents must use lower case for all HTML element and attribute names. This difference is necessary because XML is case-sensitive e.g. <li> and <LI> are considered to be different tags.
In SGML-based HTML 4.0 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.
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>
XML does not support attribute minimization. Attribute-value
pairs must be written in full. Attribute names such as compact and
checked cannot occur in elements without their value being
specified.
CORRECT: unminimized attributes
<dl compact="compact">
INCORRECT: minimized attributes
<dl compact>
Empty elements must end with />. For instance, <br />
or <hr />.
CORRECT: terminated empty tags
<br /><hr />
INCORRECT: unterminated empty tags
<br><hr>
XHTML alters the HTML 4.0 rules for the treatment of whitespace in attribute values. In particular, XHTML strips leading and trailing white space, and maps sequences of one or more white space characters (including line breaks) to a single inter-word space (an ASCII space character for western scripts). See Section 3.3.3 of [XML].
In XHTML, the script and style elements are declared as having
#PCDATA content. As a result entities such as < and
& will be expanded 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.
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 4.0 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.
The current HTML 4.0 DTDs do not reflect errata changes made to the HTML 4.0 Recommendation [HTML]. The XHTML DTDs incorporate these errata, and thus errors in HTML 4.0 DTDs are corrected in the XHTML DTDs. The errata can be found at [ERRATA].
HTML Tidy is W3C sample code that automatically converts existing web content to XHTML. It can cope with a wide range of markup errors, and offers a means to smoothly transition existing HTML documents to XHTML. For more information, see [TIDY].
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.
An XHTML document may be transmitted using one of the following
Internet Media Types. Using these types, document authors can
create portable internet content by creating XHTML documents that
can be served to generic XML applications (text/xml),
to legacy HTML user agents (text/html), and to new
XHTML applications (text/xhtml).
text/xmlSince any XHTML document is also a well-formed and valid XML
document, it may be transmitted using the Internet Media Type
text/xml [RFC2376]. However, transmitting an XHTML document as
text/xml loses information in two ways:
First, constraints given in the text of this specification that are not captured by the XHTML DTD associated with the document cannot be enforced by a generic XML validating parser.
Second, rendering semantics described in the HTML 4.0 specification that are not captured by the style sheet associated with the document cannot be rendered by a generic XML application.
In short, if all the recipient knows is text/xml,
it cannot know to check XHTML-specific parsing constraints, and it
cannot know to render XHTML-specific semantics.
A server might still choose to transmit an XHTML document as
text/xml, in those circumstances where only generic
XML processing on the document is required.
One of the ultimate goals for future versions of XHTML is that
there be no such information to lose by transmitting an XHTML
document as text/xml. However, this specification does
not yet accomplish that goal.
text/htmlIf an XHTML document conforms to the guidelines contained in Appendix C, it is also an HTML 4.0
document, and so it may be transmitted using the Internet Media Type
text/html. However, transmitting an XHTML document as
text/html loses information in two ways:
First, the recipient has no way to know that the document claims to be a valid XML document, and so it cannot use generic XML tools to check the syntax of the document or to render the document. It must therefore be prepared to check HTML 4.0 syntax and/or render HTML 4.0 semantics in an ad hoc fashion.
Second, the recipient has no way to know that the document claims to be a valid XHTML document, and so it cannot enforce constraints required of an XHTML document that are not required of an HTML 4.0 document.
A server might still choose to transmit an XHTML document as
text/html, in those circumstances where XHTML support
is not present in the recipient.
Transmitting an XHTML document using the Internet Media Type
text/html will help support a smooth transition from
HTML to XHTML and encourage its early adoption. An XHTML document
transmitted using this type is likely to be processed in the usual
way be existing user agents.
text/xhtmlSince each of the above Internet Media Types
text/xml and text/html discard information
about an XHTML document, it is the intention of the W3C to register
the Internet Media Type text/xhtml.
Conforming user agents that encounter a document of type
text/xhtml may assume that the document claims to conform to
this specification. This assumption means that the recipient of a
document of type text/xhtml must check the document
for well-formedness and may check the document for validity against
the XHTML DTD associated with the document.
We very much welcome discussion on the role of this media type and alternative mechanisms.
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 subsetting of XHTML 1.0 in a uniform way through the definition of new modules.
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.
Modularization brings with it several advantages:
It provides a formal mechanism for subsetting 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.
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.
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.
These DTDs approximate the HTML 4.0 DTDs. It is likely that when the DTDs are modularized, a method of DTD construction will be employed that corresponds more closely to HTML 4.0.
The XHTML entity sets are the same as for HTML 4.0, 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.
This appendix is normative.
The following elements have prohibitions on which elements they can contain (see Section 4.1.9). 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, or iframe
elements. |
label |
cannot contain other label elements. |
form |
cannot contain other form elements. |
This appendix is informative.
This appendix summarizes design guidelines for authors who wish their XHTML documents to render on existing HTML user agents.
Be aware that processing instructions are rendered on some user agents.
Include a space before the trailing / and
> of empty elements, e.g.
<br />,
<hr /> and <img
src="karen.jpg" alt="Karen" />. Also, use the
minimized tag syntax for empty elements, e.g. <br />, as the alternative syntax <br></br> allowed by XML gives
uncertain results in many existing user agents.
Given an empty instance of an element whose content model is not
EMPTY (for example, an empty title or paragraph) do
not use the minimized form (e.g. use
<p> </p> and not
<p />).
Use external style sheets if your style sheet uses
< or & or ]]>. Use
external scripts if your script uses < or
& or ]]>.
Avoid line breaks and multiple white space characters within attribute values. These are handled inconsistently by user agents.
Use both the lang and xml:lang
attributes when specifying the language of an element.
In XML, URIs 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 4.0. Many existing HTML clients
don't support the use of ID-type attributes in this way,
so if you want to be able to process the document on HTML clients,
you may wish to supply both id and name values
on the target element, e.g., <a id="foo"
name="foo">...</a>
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"' />).
This appendix is informative.