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

The mask-border-width CSS property sets the width of an element's mask border.


/* Keyword value */
mask-border-width: auto;

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

/* <percentage> value */
mask-border-width: 25%;

/* <number> value */
mask-border-width: 3;

/* vertical | horizontal */
mask-border-width: 2em 3em;

/* top | horizontal | bottom */
mask-border-width: 5% 15% 10%;

/* top | right | bottom | left */
mask-border-width: 5% 2em 10% auto;

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

The mask-border-width property may be specified using one, two, three, or four values chosen from the list of values below.

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


The width of the mask border, specified as a <length> or a <percentage>. Percentages are relative to the width of the border area for horizontal offsets and the height of the border area for vertical offsets. Must not be negative.
The width of the mask border, specified as a multiple of the corresponding border-width. Must not be negative.
The width of the mask border is made equal to the intrinsic width or height (whichever is applicable) of the corresponding mask-border-slice. If the image does not have the required intrinsic dimension, the corresponding border-width is used instead.

Formal syntax

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

<length-percentage> = <length> | <percentage>


Specification Status Comment
CSS Masking Module Level 1
The definition of 'mask-border-width' in that specification.
Candidate Recommendation Initial definition
Initial value auto
Applies to all elements; In SVG, it applies to container elements excluding the <defs> element and all graphics elements
Inherited no
Percentages relative to width/height of the mask border image area
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.