Markup should be formally defined. With this formal definition of markup, external programs can be used to process the document. In a GML, a tag is attached to text elements, and formatting rules are associated with these tags. A formatter processes the text and produces a document in a format that is suitable for the output device. This application of meaningful names to tags allows the automatic processing of the document, such as the compilation of an index of tagged words.
Generalized markup allows for much flexibility.
To change the appearance of the document, it is necessary only to modify an external procedure that processes the document. This single modification will suffice for all occurrences of the appearance change, and the labor involved with hundreds or thousands of manual changes can be avoided. XML XML is a coding system that allows any type of information to be delivered across the Web. XML shall be straightforwardly usable over the Internet. XML shall support a wide variety of applications.