HTML is usually extended by innovations in a single browser, usually Microsoft's Internet Explorer or America Online's Netscape Communicator, and these changes gradually make their way into other browsers. Inevitably, the implementations are different enough that Web authors have a tough time making their sites viewable from different browsers, much less older versions of those browsers. In the last couple of years, XML has been taking the Web by storm. Whereas HTML formats and presents information, XML marks up data so that the individual pieces of information on a Web page are identified as being of a particular type. Without XML, these would be just two character strings in a sea of text on a Web page.
Industries can band together and create namespaces that facilitate the exchange of information.
This facilitates exchanging data between applications and computer systems, limiting the need for expensive, complex data-conversion programs. This new standard promises to make the Web more adaptable to existing and future uses, without forcing a wholesale redesign of millions of Web sites. Backward compatibility was a major concern of the W3C committee, and by staying faithful to the HTML 4.0 standard but extending it with XML, the consortium has created a very flexible technology for building Web sites. The second major advantage is portability, sometimes referred to as interoperability. Most Internet access is through browsers on desktop computers, though more and different types of devices are being introduced.