XHTM
is a family of XML markup languages that mirror or extend versions of the widely used Hypertext Markup Language (HTML), the language in which web pages are written.
While HTML (prior to HTML5) was defined as an application of Standard Generalized Markup Language (SGML), a very flexible markup language framework, XHTML is an application of XML, a more restrictive subset of SGML. Because XHTML documents need to be well-formed, they can be parsed using standard XML parsers—unlike HTML, which requires a lenient HTML-specific parser.
XHTML 1.0 became a World Wide Web Consortium (W3C) Recommendation on January 26, 2000. XHTML 1.1 became a W3C Recommendation on May 31, 2001. XHTML5 is undergoing development as of September 2009, as part of the HTML5 specification.
Overview
XHTML 1.0 is "a reformulation of the three HTML 4 document types as applications of XML 1.0". The World Wide Web Consortium (W3C) also continues to maintain the HTML 4.01 Recommendation, and the specifications for HTML5
and XHTML5 are being actively developed. In the current XHTML 1.0
Recommendation document, as published and revised to August 2002, the
W3C commented that, "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."
However, in 2005, the Web Hypertext Application Technology Working Group
(WHATWG) formed, independently of the W3C, to work on advancing
ordinary HTML not based on XHTML. The WHATWG eventually began working on
a standard that supported both XML and non-XML serializations, HTML5,
in parallel to W3C standards such as XHTML 2. In 2007, the W3C's HTML
working group voted to officially recognize HTML5 and work on it as the
next-generated HTML standard.
In 2009, the W3C allowed the XHTML 2 Working Group's charter to expire,
acknowledging that HTML5 would be the sole next-generation HTML
standard, including both XML and non-XML serializations. Of the two serializations, the W3C suggests that most authors use the HTML syntax, rather than the XHTML syntax.
Motivation
XHTML was developed to make HTML more extensible and increase interoperability with other data formats. HTML 4 was ostensibly an application of Standard Generalized Markup Language
(SGML); however the specification for SGML was complex, and neither web
browsers nor the HTML 4 Recommendation were fully conformant to it. The XML standard, approved in 1998, provided a simpler data format closer in simplicity to HTML 4. By shifting to an XML format, it was hoped HTML would become compatible with common XML tools; servers and proxies would be able to transform content, as necessary, for constrained devices such as mobile phones. By utilizing namespaces, XHTML documents could provide extensibility by including fragments from other XML-based languages such as Scalable Vector Graphics and MathML. Finally, the renewed work would provide an opportunity to divide HTML into reusable components (XHTML Modularization) and clean up untidy parts of the language.
Relationship to HTML
There are various differences between XHTML and HTML. The Document Object Model
is a tree structure that represents the page internally in
applications, and XHTML and HTML are two different ways of representing
that in markup (serializations). Both are less expressive than the DOM
(for example, "--" may be placed in comments in the DOM, but cannot be
represented in a comment in either XHTML or HTML), and generally XHTML's
XML syntax is a little more expressive than HTML (for example,
arbitrary namespaces are not allowed in HTML). So, firstly one source of
differences is immediate: XHTML uses an XML syntax, while HTML uses a
pseudo-SGML
syntax (officially SGML for HTML 4 and under, but never in practice,
and standardised away from SGML in HTML5). Secondly however, because the
expressible contents of the DOM in syntax are slightly different, there
are some changes in actual behavior between the two models.
First, there are some differences in syntax:
- Broadly, the XML rules require that all elements be closed, either by a separate closing tag or using self-closing syntax (e.g.
<br />), while HTML syntax permits some elements to be unclosed because either they are always empty (e.g. <input>) or their end can be determined implicitly ("omissibility", e.g. <p>).
- XML is case-sensitive for element and attribute names, while HTML is not.
- Some shorthand features in HTML are omitted in XML, such as (1) attribute minimization, where attribute values or their quotes may be omitted (e.g.
<option selected> or <option selected=selected>, while XML this must be expressed as <option selected="selected">); (2) element minimization may be used to remove elements entirely (such as <tbody>
inferred in a table if not given); and (3) the rarely used SGML syntax
for element minimization ("shorttag"), which most browsers do not
implement.
- There are numerous other technical requirements surrounding
namespaces and precise parsing of whitespace and certain characters and
elements. The exact parsing of HTML in practice has been undefined until
recently; see the HTML5 specification ([HTML5]) for full details, or the working summary (HTML vs. XHTML).
Secondly, in contrast to these minor syntactical differences, there
are some behavioral differences, mostly arising from the underlying
differences in serialization. For example:
- Most prominently, behavior on parse errors differ. A fatal parse
error in XML (such as an incorrect tag structure) causes document
processing to be aborted.
- Most content requiring namespaces will not work in HTML, except the
built-in support for SVG and MathML in the HTML5 parser along with
certain magic prefixes such as
xlink.
- JavaScript processing is a little different in XHTML, with minor
changes in case sensitivity to some functions, and further precautions
to restrict processing to well-formed content. Scripts must not use the
document.write() method; it is not available for XHTML. The innerHTML
property is available, but will not insert non-well-formed content. On
the other hand, it can be used to insert well-formed namespaced content
into XHTML.
- CSS is also applied slightly differently. Due to XHTML's
case-sensitivity, all CSS selectors become case sensitive for XHTML
documents. Some CSS properties, such as backgrounds, set on the
<body> element in HTML are 'inherited upwards' into the <html> element; this appears not to be the case for XHTML.
Adoption
The similarities between HTML 4.01 and XHTML 1.0 led many web sites and content management systems
to adopt the initial W3C XHTML 1.0 Recommendation. To aid authors in
the transition, the W3C provided guidance on how to publish XHTML 1.0
documents in an HTML-compatible manner, and serve them to browsers that
were not designed for XHTML.
Such "HTML-compatible" content is sent using the HTML media type (text/html) rather than the official Internet media type for XHTML (application/xhtml+xml).
When measuring the adoption of XHTML to that of regular HTML,
therefore, it is important to distinguish whether it is media type usage
or actual document contents that is being compared.
Most web browsers have mature support for all of the possible XHTML media types. The notable exception is Internet Explorer versions 8 and earlier by Microsoft; rather than rendering application/xhtml+xml
content, a dialog box invites the user to save the content to disk
instead. Both Internet Explorer 7 (released in 2006) and Internet
Explorer 8 (released in March 2009) exhibit this behavior. Microsoft developer Chris Wilson explained in 2005 that IE7’s priorities were improved browser security and CSS support, and that proper XHTML support would be difficult to graft onto IE’s compatibility-oriented HTML parser; however, Microsoft added support for true XHTML in IE9.
As long as support is not widespread, most web developers avoid using XHTML that is not HTML-compatible, so advantages of XML such as namespaces, faster parsing and smaller-footprint browsers do not benefit the user.
Criticism
In the early 2000s, some web developers began to question why Web authors ever made the leap into authoring in XHTML.
Others countered that the problems ascribed to the use of XHTML could
mostly be attributed to two main sources: the production of invalid
XHTML documents by some Web authors and the lack of support for XHTML
built into Internet Explorer 6.
They went on to describe the benefits of XML-based Web documents (i.e.
XHTML) regarding searching, indexing and parsing as well as
future-proofing the Web itself.
In October 2006, HTML inventor and W3C chair Tim Berners-Lee,
introducing a major W3C effort to develop a new HTML specification,
posted in his blog that, "The attempt to get the world to switch to XML …
all at once didn't work. The large HTML-generating public did not move …
Some large communities did shift and are enjoying the fruits of
well-formed systems … The plan is to charter a completely new HTML
group."
The current HTML5 working draft says "special attention has been given
to defining clear conformance criteria for user agents in an effort to
improve interoperability … while at the same time updating the HTML
specifications to address issues raised in the past few years." Ian Hickson, editor of the HTML5 specification criticising the improper use of XHTML in 2002, is a member of the group developing this specification and is listed as one of the co-editors of the current working draft.
Simon Pieters researched the XML-compliance of mobile browsers and concluded “the claim that XHTML would be needed for mobile devices is simply a myth”.
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CSS
is a style sheet language used for describing the presentation semantics (the look and formatting) of a document written in a markup language. Its most common application is to style web pages written in HTML and XHTML, but the language can also be applied to any kind of XML document, including plain XML, SVG and XUL.
CSS is designed primarily to enable the separation of document
content (written in HTML or a similar markup language) from document
presentation, including elements such as the layout, colors, and fonts. This separation can improve content accessibility,
provide more flexibility and control in the specification of
presentation characteristics, enable multiple pages to share formatting,
and reduce complexity and repetition in the structural content (such as
by allowing for tableless web design).
CSS can also allow the same markup page to be presented in different
styles for different rendering methods, such as on-screen, in print, by
voice (when read out by a speech-based browser or screen reader) and on Braille-based, tactile
devices. It can also be used to allow the web page to display
differently depending on the screen size or device on which it is being
viewed. While the author of a document typically links that document to a
CSS file, readers can use a different style sheet, perhaps one on their
own computer, to override the one the author has specified.
CSS specifies a priority scheme to determine which style rules apply
if more than one rule matches against a particular element. In this
so-called cascade, priorities or weights are calculated and assigned to rules, so that the results are predictable.
The CSS specifications are maintained by the World Wide Web Consortium (W3C). Internet media type (MIME type) text/css is registered for use with CSS by RFC 2318 (March 1998), and they also operate a free CSS validation service.
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