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XML is the Extensible Markup Language. This improves the functionality of the Web through letting you recognize your information in a more precise, adaptable and flexible way.
This is extensible as it is not a fixed format as HTML (that is a single, predefined markup language). In its place, XML is actually a metalanguage a language for exapling other languages that lets you design your own markup languages for limitless various types of documents. XML can perform this as it's written within SGML, ISO 8879 (international standard metalanguage for text document markup).