The HTML-language is an Interpreter language for formating especially text and images on a screen.
The Browser (Netscape, Explorer, Amaya, ...) serves as the Interpreter for this language. But for a large number of special applications images and "plain text" are not very conveniant to provide the necessary information in the internet.
To increase the number of displayable objects, a new language was created: XML. It contains all elements of HTML (downwards compatibility) and a number of new elements for e.g. music notes, multi-layer-design and mathematical formulars. Especially the last element is very important for the natural scientific community since in HTML more complicating formulars can only be included as images. A group of users called W3C designed a List of Recommendation how a mathematical "HTML"-language should look like. This list has been officially accepted and all commands will be includes into furthe versions of all browsers.
But until now no good Browsers for XML exist.
The standard browsers until now do not support XML at all or only in parts.
Special Browsers for XML as e.g. AMAYA are extremely slow and quite buggy.