Question:

What is XML?

Answer:

XML is the 'Extensible Markup Language' (extensible because it is not a fixed format like HTML). It is designed to enable the use of SGML on the World Wide Web.

XML is not a single, predefined markup language: it's a metalanguage-a language for describing other languages-which lets you design your own markup. (A predefined markup language like HTML defines a way to describe information in one specific class of documents: XML lets you define your own customized markup languages for different classes of document.) It can do this because it's written in SGML, the international standard metalanguage for markup.

Question:

Why should I use XML instead of HTML?

Answer:

Authors and providers can design their own document types using XML, instead of being stuck with HTML. Document types can be explicitly tailored to an audience, so the cumbersome fudging that has to take place with HTML to achieve special effects should become a thing of the past: authors and designers will be free to invent their own markup elements;

Information content can be richer and easier to use, because the hypertext linking abilities of XML are much greater than those of HTML.

XML can provide more and better facilities for browser presentation and performance;

It removes many of the underlying complexities of SGML in favor of a more flexible model, so writing programs to handle XML will be much easier than doing the same for full SGML.

Information will be more accessible and reusable, because the more flexible markup of XML can be used by any XML software instead of being restricted to specific manufacturers as has become the case with HTML.

Valid XML files are kosher SGML, so they can be used outside the Web as well, in an SGML environment.

Question:

Where can I get an XML browser?

Answer:

Remember the XML specification is still new, so a lot of what you see now is experimental. As with HTML, there won't be just one browser, but many. However, because the potential number of different XML applications is not limited, no single
browser can be expected to handle 100% of everything.

Some of the generic parts of XML (eg parsing, tree management, searching, formatting, etc) are being combined into general-purpose browser libraries or toolkits to make it easier for developers to take a consistent line when writing XML
applications. Such applications can then be customized by adding semantics for specific markets, or using languages like Java to develop plugins for generic browsers and have the specialist modules delivered transparently over the Web.

MSIE5 handles XML but currently still renders it via CSS, using a largely HTML-derived model, so not all the stylesheet options work. Microsoft are also the architects of a hybrid solution in which you can embed fragments of XML in HTML files because current HTML-only browsers simply ignore element markup which they don't recognize.

The publicly-released Netscape code (Mozilla) has resulted in a test XML implementation including an application of RDF plus James Clark's expat XML parser.

The authors of the MultiDoc Pro SGML browser, CITEC, have joined forces with Mozilla to produce a multi-everything browser called DocZilla, which reads HTML, XML, and SGML, with XSL and CSS stylesheets. This runs under NT and linux and is currently Alpha. See http://www.doczilla.org for details. This is early alpha, but is by far the most ambitious, and the only one so far backed by solid SGML expertise.

See also the notes on software for authors and developers, and the more detailed list on the XML pages in the SGML Web site at http://www.oasis-open.org/cover/xml.html.