A markup language is a set of words and symbols for describing the identity or function of the component parts of a document (for example ‘this is a paragraph’, ‘this is a heading’, ‘this is a list’, ‘this is the caption of this figure’, etc). Programs can use markup with a stylesheet to transform the document into output for screen, print, audio, video, Braille, or reprocessable data formats.

Some markup languages only describe appearances instead (‘this is italics’, ‘this is bold’, ‘this has 3mm space below’, etc), so these systems can only be used for display, and are not easily re-usable for anything else.

XML is sometimes referred to as ‘self-describing’ because the names of the markup elements can represent the type of content they hold (Example: title, chapter, link, etc).

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