This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The mask-border-outset CSS property specifies the distance by which an element's mask border is set out from its border box.


/* <length> value */
mask-border-outset: 1rem;

/* <number> value */
mask-border-outset: 1.5;

/* vertical | horizontal */
mask-border-outset: 1 1.2;

/* top | horizontal | bottom */
mask-border-outset: 30px 2 45px;

/* top | right | bottom | left */
mask-border-outset: 7px 12px 14px 5px;

/* Global values */
mask-border-outset: inherit;
mask-border-outset: initial;
mask-border-outset: unset;

The mask-border-outset property may be specified as one, two, three, or four values. Each value is a <length> or <number>. Negative values are invalid.

  • When one value is specified, it applies the same outset to all four sides.
  • When two values are specified, the first outset applies to the top and bottom, the second to the left and right.
  • When three values are specified, the first outset applies to the top, the second to the left and right, the third to the bottom.
  • When four values are specified, the outsets apply to the top, right, bottom, and left in that order (clockwise).


The size of the mask border outset as a dimension.
The size of the mask border outset as a multiple of the corresponding border-width.

Formal syntax

[ <length> | <number> ]{1,4}


Specification Status Comment
CSS Masking Module Level 1
The definition of 'mask-border-outset' in that specification.
Candidate Recommendation Initial definition
Initial value 0
Applies to all elements; In SVG, it applies to container elements excluding the <defs> element and all graphics elements
Inherited no
Media visual
Computed value as specified, but with relative lengths converted into absolute lengths
Animation type discrete
Canonical order per grammar

Browser compatibility


© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.