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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 value0
Applies toall elements; In SVG, it applies to container elements excluding the <defs> element and all graphics elements
Computed valueas specified, but with relative lengths converted into absolute lengths
Animation typediscrete
Canonical orderper grammar

Browser compatibility


Document Tags and Contributors

 Contributors to this page: mfluehr
 Last updated by: mfluehr,