• A system for annotating a document in a way that is syntactically distinguishable from the text. The idea and terminology evolved from the “marking up” of paper manuscripts (i.e., the revision instructions by editors), which is traditionally written with a red or blue pencil on authors’ manuscripts. In digital media, this “blue pencil instruction text” was replaced by tags, which indicate what the parts of the document are, rather than the details of how they might be shown on some display. This lets authors avoid formatting every instance of the same kind of thing redundantly (and possibly inconsistently). It also avoids the specification of fonts and dimensions which may not apply to many users (such as those with different-size displays, impaired vision, and screen-reading software). Some markup languages, such as the widely used HyperText Markup Language (HTML), have pre-defined presentation semantics. Others, such as XML and its predecessor SGML, allow but do not impose such prescriptions—all the while allowing users to define any custom document components as they wish. ← Wikipedia

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