XML for eXtensible Markup Language, it's like HTML language allowing defining new markup. It is effectively about a language allowing structuring various documents thanks to markup.
Contrary to HTML, which is to be considered as a defined and fixed language (with a limited number of markup), XML can be considered as a metalanguage allowing to define the other languages, that is to define new markup allowing to describe the presentation of a text.
The power of XML lives in its capacity to be able to describe any domain of data thanks to its extensibility. It is going to allow to structure, to put the vocabulary and the syntax of the data which it is going to contain.
Really XML markup describes the contents rather than the presentation (contrary to HTML). So, XML allows separating the contents of the presentation that allows for example to post the same document on applications without requiring for all to create so many versions of the document which we require of representations!

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