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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.^[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.^[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.

MathML is a markup language for mathematical notation, structure and content on the web. Its elements describe expressions within a document rather than reducing a formula to a flat character string. MathML Core specifies a subset suited to web browser implementation; the cited 24June2025 document is a Candidate Recommendation Snapshot.^[ref]

Consider the typeset fraction $\frac{x+1}{2}$. A presentation encoding is <math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><mi>x</mi><mo>+</mo><mn>1</mn></mrow><mn>2</mn></mfrac></math>. The mi, mo and mn elements identify a symbolic name, an operator and numbers. The mrow groups the numerator; the two children of mfrac provide the numerator and denominator.^[ref]

Grouping changes the represented expression. At $x=3$, the intended fraction gives $(3+1)/2=2$, whereas $x+1/2=7/2$. This arithmetic illustrates the meaning assigned to the structure; MathML presentation markup does not itself execute the calculation. Maintaining a structured representation helps document tools manipulate a formula and lets layout follow its mathematical grouping.