Nyhetsbrev maj 1999

Innehåll

W3Cs Rekommendation 5 maj 1999
XHTML

W3C Rekommendation 5 Maj 1999

Riktlinjer för utformning av innehåll på webben, version 1.0 finns i en preliminär svensk översättning här:

http://w3c.sics.se/WAI-WEBCONTENT.html

XHTML(tm): The New HTML

Steven Pemberton

In February 1998, the World Wide Web Consortium (W3C), an international industry consortium with more than 300 members, issued the definition of XML, a subset of the meta-language SGML, the language that had originally been used to define HTML.

SGML is a difficult language to fully implement, and so the goal of XML is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. XML has been designed for ease of implementation and for interoperability with both SGML and HTML.

For some time HTML had been growing by accretion of elements added by different vendors which had caused problems with interoperability between browsers, since browsers had to be updated to accept the new elements, and this didn't always happen.

XML allows you to define and use your own HTML-like languages with fewer interoperability problems. The earlier release of the first Cascading Style Sheets (CSS) definition in 1996 means that you can also easily define how your new language should appear on the screen.

Now you can serve documents like this:

<name>Steven Pemberton</name>

<street>Kruislaan 413</street>

<city>Amsterdam</city>

</address>

with several different style sheets for different presentation styles.

So the question arises, if you can now define your own languages in an interoperable way, is there still a need for HTML?

It was with this question that we organised an international workshop in May 1998 in San Francisco. The consensus of the attendees was that there was a need for a new version of HTML, for several reasons:

The community of web-page authors appreciate the existence of a standard markup, HTML needed to be cleaned up (to get rid of historical flotsam)
Basing the new HTML on XML would make it easier to implement, and ensure better quality markup
If HTML could be modularised it would allow other XML applications to use parts, and would allow special purpose devices like portable phones to use a subset
Some required new functionality, principally for forms, could be added.

So the W3C HTML Working Group was reformed with a mission to develop the next generation of HTML as a suite of XML tag sets with a clean migration path from HTML 4.0. Some of the expected benefits include:

reduced authoring costs, an improved match to database & workflow applications, a modular solution to the increasingly disparate capabilities of browsers, and the ability to cleanly integrate HTML with other XML applications.

Recently a public working draft of the first version of the new HTML, which now carries the (trade-marked) name XHTML, was released. This version is just the migrated version of HTML 4.0; the tasks of modularising, profiling (subsetting and extending), and defining new forms are parallel ongoing work items.

Migrating HTML from SGML to XML meant making some decisions because of differences between the two meta-languages. One of these is that XMLis case-sensitive, where (old) HTML was not: this means that elements in XHTML must be written in lower case. Other XML differences mean that all elements must now be closed, such as using <li> ...</li> for list items, and <p>...</p> for paragraphs, and empty elements must be written specially, such as <br/> and <hr/> instead of <br> and <hr>.

Despite these differences, it is still possible, by following a few authoring guidelines, to create documents that are compatible with most old-HTML browsers, giving the advantage of a smooth changeover from SGML-based to XML-based browsers. New generations of XML-enabled browsers, such as Microsoft Internet Explorer 5, and Netscape 5 are now beginning to emerge.

The expectation is that XHTML 1.0 will become a W3C Recommendation shortly; modularisation, profiling, and new forms will then follow later in the year.

References:

W3C: http://www.w3.org/

HTML: http://www.w3.org/TR/REC-html40/

XML: http://www.w3.org/TR/1998/REC-xml-19980210

CSS: http://www.w3.org/TR/REC-CSS1 and http://www.w3.org/TR/REC-CSS2

XHTML: http://www.w3.org/TR/WD-html-in-xml/

XHTML Working Group: http://www.w3.org/Markup/

Steven Pemberton is a researcher at the CWI, and chair of the W3C HTML Working Group.
Steven.Pemberton@cwi.nl
http://www.cwi.nl/~steven