flex is a new value added to the CSS display property. Along with inline-flex it causes the element that it applies to to become a flex container, and the element's children to each become a flex item. The items then participate in flex layout, and all of the properties defined in the CSS Flexible Box Layout Module may be applied.

The flex property is a shorthand for the flexbox properties flex-grow, flex-shrink and flex-basis.

In addition <flex> can refer to a flexible length in CSS Grid Layout.

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Property reference

Further reading