= Markup language
A <markup language> describes annotations embedded in or associated with document content. Its syntax distinguishes ordinary content from instructions identifying structure, meaning or presentation. <HTML> is a concrete example: a paragraph and an emphasized phrase can be represented explicitly instead of inferred from their appearance.<HTML Standard by WHATWG>[Introduction, Background and A quick introduction to HTML]{ref}
For example, `<p>Save <em>your work</em>.</p>` identifies a paragraph containing an emphasized phrase. The angle-bracketed <HTML tags> mark elements; the words between them supply content. A <web browser> can build a <Document Object Model> from this source, then use <CSS> to choose its presentation. Changing the emphasis styling need not change which phrase is structurally emphasized.
The syntax needs a way to represent its own reserved characters as content. In <HTML>, `<` can represent a literal less-than character in text rather than starting a tag. A document explaining markup therefore needs to distinguish a displayed example from active markup that the parser interprets as document structure.<HTML Standard by WHATWG>[The HTML syntax, Character references; parsing text versus markup]{ref}
A <markup language> is useful when documents must be transformed, searched or rendered according to their structure. It does not follow that every source annotation creates a visible box, or that a visible line break implies a particular structural element. Source syntax, the parsed document and the final rendering are related representations with different jobs.
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