CSS mask-border-outset Property Reference

The CSS mask-border-outset property is a standard property in CSS Masking. Review its formal syntax, computed behavior, and practical declarations.

Updated 2026-08-12

mask-border-outset overview

The CSS mask-border-outset property is a standard property in CSS Masking. 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

mask-border-outset formal syntax

Field	Value
Formal syntax	[ <length> \| <number> ]{1,4}
Initial value	0
Applies to	allElementsSVGContainerElements
Inherited	No
Percentages	no
Computed value	asSpecifiedRelativeToAbsoluteLengths
Animation type	discrete
Media	visual
Specification group	CSS Masking
Status	Standard

Practical examples

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

mask-border-outset live example

CSS

.reset-example {
  mask-border-outset: initial;
}

.inherit-example {
  mask-border-outset: inherit;
}

@supports (mask-border-outset: initial) {
  .supported {
    mask-border-outset: initial;
  }
}