e HTML will continue to be used for standard web pages, XML will be useful for applications that need more intelligent documents and more processing ability (DWP - Pg. 745).
The main limitations of HTML are lack of extensibility, structure, and validation. (http:// metalab.unc.edu/pub/sun-info/standards/xml/why/xmlapps.html).
Extensibility. HTML has a fixed number of tags. While the W3C and browser developers can (and frequently do) add tags, users cannot create their own tags to more accurately describe their data.
Structure. HTML does not support the creation of nested tags, which would be used to describe and represent databases or object hierarchies.
Validation. HTML does not support document validation. It has no means of allowing an application to check the data for validity, or to ensure that the markup is correct and well formed.
XML differs from HTML in all three of these major areas:
It allows developers to define new tags and attributes as needed
It allows document tags to be nested as deeply as needed.
Any XML document can include or make reference to a description of its grammar and syntax for use by applications that need to validate the structure of a document.
XML will be most widely used in applications, which cannot be accomplished within the limitations of HTML. According to Jon Bosak of Sun Microsystems (Future of the web)
"These applications can be divided into four broad categories:
Applications that require the Web client to mediate between two or more heterogeneous databases
Applications that require the Web client to present different views of the same data to different users.
Applications that require the Web client to present different views of the same data to different users.
Applications in which intelligent Web agents attempt to tailor information discovery to the needs of the individual users.
One more quote from Mr. Bosak: "XML can do for data what Java has done ...