 CSS maximum size

A CSS maximum size is an upper constraint supplied by a maximum-size property such as max-width or max-height. It participates in finding the CSS used value, together with the CSS preferred size, CSS minimum size and applicable layout rules.^[ref]

For a simple block with zero padding and borders, width: 500px; max-width: 400px; min-width: 240px gives a used width of $400$ CSS pixels. With the same constraints but preferred width $180$, the used width is $240$. Once these fixed dimensions are resolved, this simplified calculation iswhere $p$ is the tentative width, $m$ the minimum and $M$ the maximum. It expresses the order of limiting the tentative width and then enforcing the minimum in this case.^[ref]

If the minimum exceeds the maximum, the minimum wins: min-width: 320px; max-width: 200px does not squeeze the box below its $320$-pixel minimum. Also check CSS box-sizing before interpreting which edge the numbers measure.

A maximum constraint limits the box's size; it does not guarantee that every unbroken descendant fits or specify clipping. Such content may cause CSS overflow. Percentage constraints likewise require the appropriate CSS containing block and a usable dimension reference. These qualifications make max-width: 100% a sizing instruction rather than a universal promise that nothing can extend outside a container.