3. DocBook !

DocBook is a DTD - Document Type Definition. now what is this? well say for example having said XML is in itself a rule set , suppose i use an markup element tag <author> in my document and another author uses <writer> element tag in his document, isn't it true we are trying to convey the same meaning .Imagine thousands of pages are being written for the web, for the publishing industry and what a waste of enormous time if people would like to convey similar meaning but use different elements with the core language being same, this is where an DTD comes into picture.

                       Docbook DTD is a very popular set of tags for describing books, 
                       articles and other prose documents, particularly technical documentation. Docbook 
                       is defined using the native DTD syntax of SGML 
                       and XML. Like HTML, DocBook is an example of 
                       a markup language defined in SGML/XML.
 ---From the book DocBook - The Definitive Guide by Norman Walsh and Leonard Muellner.