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Section A.1. What Is XML?

A.1. What Is XML?

XML (Extensible Markup Language) is an Internet-friendly format for data and documents, invented by the World Wide Web Consortium (W3C). "Markup" denotes a way of expressing the structure of a document within the document itself. XML has its roots in a markup language called SGML (Standard Generalized Markup Language), which is used in publishing and shares this heritage with HTML. XML was created to do for machine-readable documents on the Web what HTML did for human-readable documents — that is, provide a commonly agreed-upon syntax, so that processing the underlying format becomes a commodity and documents are made accessible to all users.

Unlike HTML, though, XML comes with very little predefined. HTML developers are accustomed to both the notion of using angle brackets (< >) for denoting elements (that is, syntax), and also the set of element names themselves (such as head, body, etc.). XML shares only the former feature (i.e., the notion of using angle brackets for denoting elements). Unlike HTML, XML has no predefined elements, but is merely a set of rules that lets you write other languages like HTML. (To clarify XML's relationship with SGML: XML is an SGML subset. In contrast, HTML is an SGML application. RSS uses XML to express its operations and thus is an XML application.)


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