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XML
Standards Update;
XHTML Basic Becomes a W3C Recommendation
XHTML Basic is Now a Recommendation
On December 19, 2000,
W3C posted XHTML Basic as a Recommendation.
A W3C Recommendation indicates that a specification is stable, contributes to Web interoperability, and has been reviewed by the W3C Membership, who favor its adoption by the industry.
A Recommendation is, for all intents and purposes, a
W3C "standard."
The XHTML Basic specification reflects cross-industry agreement on a set of markup language features that
allow authors to create rich Web content
for delivery to a wide range of devices, including mobile phones, personal digital assistants (PDAs), pagers, and television-based Web browsers.
But what is XHTML Basic? Why do we need
it? How is it different from HTML? Is it
different from XHTML?
As you probably know,
HTML or Hypertext Markup Language is the traditional
language for the encoding of Web pages. In January 2000, W3C published the XHTML 1.0 Recommendation, which combined the well-known features of HTML with the power of
XML. XHTML provided a mechanism to create well-formed
Web page content and to extend that content.
Because the XHTML content could be used and extended
many different ways, it became important to have a
clear modularization and extension strategy, hence
another XHTML-related specification, entitled
"Modularization of XHTML", describes a
mechanism to allow authors to mix and match content
from well-defined subsets of XHTML 1.0 elements and
attributes in a standard way.
It turns out that one
of the first ways that authors need to optimize XHTML
is to provide delivery for the new generation of Web
browsers that can be found on mobile phones, PDAs,
pagers, and Web-TV. The XHTML Basic Recommendation combines
XHTML modules in a standard manner to support delivery
to mobile Web applications. According to Tim Berners-Lee, W3C Director. "The simplicity of early versions of HTML made interoperability easy. While XHTML 1.0 is a powerful language, support for the full XHTML 1.0 feature set may be too much to expect browsers on cell phones and other small devices to handle. XHTML Basic offers the simplicity and wide interoperability of early versions of HTML and reflects ten years of Web experience, including advances in XML and
accessibility."
The motivation for XHTML Basic
was to provide
content that can be shared across multiple devices (e.g. desktop, TV, and mobile phones), and that is rich enough to be used for simple content authoring.
In theory, an XHTML Basic document can be presented on the maximum number of Web clients.
The XHTML Basic document type includes the minimal set of modules required to be an XHTML host language document type, and in addition it includes images, forms, basic tables, and object support.
As with XHTML 1.0, XHTML Basic is also designed as a common base that may be
extended.
HTML features that are not part of XHTML Basic
include:
- <style> (The style element is not supported.)
External style sheets are recommended. The link element can be used to include external style sheets.
Separation between structure and presentation allows user agents to download the style sheets if they support style sheets; user agents that do not support style sheets can ignore the external
style sheet.
- <script> and <noscript> (The script and noscript elements are not supported.)
Today most small devices have limited memory and CPU power and may not
support the execution of script programs may not be supported.
- <font>, <bold>, <italic>
(The font, bold, italic and other font related elements are not supported.)
Many simple Web clients cannot display fonts other than monospace. It is recommended that style sheets be used to create a presentation that is appropriate for the device.
- <form> (Basic XHTML forms elements are
supported but more sophisticated forms support is not present).
- <table> (Basic XHTML tables are
supported, but more sophisticated display functions are not appropriate for
mobile device displays.) Nesting of tables is not allowed.
- <frame> (Frames are not
supported.) Frames depend on a screen interface and may not be applicable to some small
devices.
Other W3C Year-End Highlights
In addition to XHTML Basic, W3C released the
specifications that make up DOM Level 2 as Recommendations on November 13,
2000. These specifications included:
Late in the month of December (12/20/00), W3C
promoted two standards to Proposed Recommendation. These
specifications include:
Also in November/December , W3C promoted 6
standards to the status of Candidate Recommendation. Notable among these
specifications are the long awaited work on XSL and SVG:
First Working Draft for XML Query Algebra
The first Working Draft of XML Query Algebra was
posted on December 4. This work builds on SQL, OQL, and nested relational algebra (NRA).
It was also influenced by systems such as Quilt, UnQL, XDuce, XML-QL, XPath, XQL, and
YaTL.
The XML Query Algebra is used to give a semantics for the
overall XML Query Language. It is therefore important that the operations of the algebra
be well-defined. The XML Query Algebra is also used to support query optimization, so the algebra should
also possess a rich set of laws. The XML Query Algebra should be powerful enough to capture the semantics of many XML query languages, and the laws
should include analogues of most of the laws of relational algebra.
In addition, it is also common for a query language to exploit schemas or
types. This is the case in SQL, OQL, and NRA. Types can be used to detect certain kinds of errors at compile time and to support query optimization.
The XML Query algebra uses a simple type system that captures the essence of [XSchema1]. The type system is close to that used in XDuce [HP2000]. On this basis, the XML Query algebra is statically typed. This allows
us to determine and check the output type of a query on documents conforming to an input type at compile time rather than at runtime.
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