It was focused on how to extend HTML in a more general fashion. The approach taken with Mosaic's IMG tag became an unfortunate precedent in how HTML was later extended. This strategy was then mirrored by Microsoft when it engaged Netscape in the browser war. Pretty soon the explosion of tags began to splinter the Web into ghettoes of pages viewable by either Navigator or Explorer, but not both. The fight over how to add image support to HTML was a fight about abstraction.
Abstraction is a key concept for understanding just what XML is designed to do.
Where HTML grew presentation-specific tags like barnacles, XML is designed as a framework , wherein any tag can be created but has no concept of presentation. Instead, tags denote structure alone, and presentation is left to another application. Let's take another step back to see how this is important. One of the things that makes the Web so cool is that data created for it is accessible from any kind of computer and, with work, any number of networked devices. This device independence is crucial to the Web's success, because it means that any page marked up in HTML can be viewed on any platform and by any application that knows how to parse the language.