CSS box-flex Property Reference

The CSS box-flex property is a nonstandard property in Mozilla Extensions. Review its formal syntax, computed behavior, and practical declarations.

Updated 2026-08-12

box-flex overview

The CSS box-flex property is a nonstandard property in Mozilla Extensions. Review its formal syntax, computed behavior, and practical declarations.

The property is not inherited by default. Use an explicit global keyword when inheritance is intended.

Syntax and formal definition

box-flex formal syntax

Field	Value
Formal syntax	<number>
Initial value	0
Applies to	directChildrenOfElementsWithDisplayMozBoxMozInlineBox
Inherited	No
Percentages	no
Computed value	asSpecified
Animation type	discrete
Media	visual
Specification group	Mozilla Extensions
Status	Nonstandard

Practical examples

These declarations demonstrate valid ways to set box-flex. The initial and inherit global keywords remain available even when a property-specific value is preferable.

box-flex live example

CSS

.reset-example {
  box-flex: initial;
}

.inherit-example {
  box-flex: inherit;
}

@supports (box-flex: initial) {
  .supported {
    box-flex: initial;
  }
}