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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.

A stretchy mathematical operator can adapt its extent to the surrounding mathematical layout. In MathML Core, an mo element's stretchy property participates in this decision. A mathematical delimiter around a tall typeset fraction is a familiar example. Actual stretching also requires an applicable constraint and suitable computer font data.^[ref]

For an original example, use <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo stretchy="true" minsize="2em" maxsize="3em">(</mo><mfrac><mi>x</mi><mn>2</mn></mfrac><mo stretchy="true" minsize="2em" maxsize="3em">)</mo></mrow></math>. The row places two mathematical delimiters around a typeset fraction with numerator $x$ and denominator 2.

Suppose the positive block-axis target totals, already expressed at a common scale, are $1.5$, $2.5$ and $4$ em. The stated size bounds adjust them to $2$, $2.5$ and $3$ em, respectively. This is the target-size constraint step, not a prediction of exact rendered ink heights. The complete algorithm also handles ascent/descent and symmetry about the Math axis.^[ref]

Suitable mathematical glyph variants or mathematical glyph assemblies can supply the extended form. Setting stretchy="false" changes the operator's instruction without changing the fraction's two operands. Layout properties and font constructions therefore belong in a controlled comparison; enlarging every outline uniformly is not the definition of stretching.