Extended Mark Up Language

XML (Extended Markup Language): XML is an HTML-like formatting language. Misconception: The recommended advice when developing Internet protocols has been be conservative in what you send and liberal in what you receive. While in the early days this philosophy fostered interoperability, any economist could tell you the long term effect of this. It leads to a large number of slightly incompatible implementations of protocols, all of which mostly work, but none of which interopate well. For example, FTP is one of the oldest Internet protocols, and there are essentially no fully complient clients or servers, and building a fully interopable client/server is extremely difficult, requiring knowledge of all the quirks of all the popular implementations. Therefore, XML has chosen the opposite approach: compliant implementations are supposed to reject all input that isn't "well-formed", even when the intent is clear. This not only avoids interoperability errors in the long run, it also dramatically improves security by guaranteeing that there is only one way to interpret things.

The Extensible Markup Language (XML) is a general-purpose specification for creating custom markup languages. It is classified as an extensible language, because it allows the user to define the mark-up elements. XML's purpose is to aid information systems in sharing structured data, especially via the Internet, to encode documents, and to serialize data; in the last context, it compares with text-based serialization languages such as JSON and YAML.

XML began as a simplified subset of the Standard Generalized Markup Language (SGML), meant to be readable by people via semantic constraints; application languages can be implemented in XML. These include XHTML, RSS, MathML, GraphML, Scalable Vector Graphics, MusicXML, and others. Moreover, XML is sometimes used as the specification language for such application languages.

XML is recommended by the World Wide Web Consortium (W3C). It is a fee-free open standard. The recommendation specifies lexical grammar and parsing requirements.

An XML document has two correctness levels:

• Well-formed. A well-formed document conforms to the XML syntax rules; e.g. if a start-tag (< >) appears without a corresponding end-tag (), it is not well-formed. A document not well-formed is not in XML; a conforming parser is disallowed from processing it.
• Valid. A valid document additionally conforms to semantic rules, either user-defined or in an XML schema, especially DTD; e.g. if a document contains an undefined element, then it is not valid; a validating parser is disallowed from processing it.