The mask-border CSS shorthand property lets you create a mask along the edge of an element's border.

Constituent properties


/* source | slice */
mask-border: url("border-mask.png") 25;

/* source | slice | repeat */
mask-border: url("border-mask.png") 25 space;

/* source | slice | width */
mask-border: url("border-mask.png") 25 / 35px;

/* source | slice | width | outset | repeat | mode */
mask-border: url("border-mask.png") 25 / 35px / 12px space alpha;

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



The source image. See mask-border-source.


The dimensions for slicing the source image into regions. Up to four values may be specified. See mask-border-slice.


The width of the border mask. Up to four values may be specified. See mask-border-width.


The distance of the border mask from the element's outside edge. Up to four values may be specified. See mask-border-outset.


Defines how the edge regions of the source image are adjusted to fit the dimensions of the border mask. Up to two values may be specified. See mask-border-repeat.


Defines whether the source image is treated as a luminance mask or alpha mask. See mask-border-mode.

Formal definition

Initial valueas each of the properties of the shorthand:
Applies toall elements; In SVG, it applies to container elements excluding the <defs> element and all graphics elements
Percentagesas each of the properties of the shorthand:
Computed valueas each of the properties of the shorthand:
Animation typeas each of the properties of the shorthand:
Creates stacking contextyes

Formal syntax

mask-border = 
<'mask-border-source'> ||
<'mask-border-slice'> [ / <'mask-border-width'>? [ / <'mask-border-outset'> ]? ]? ||
<'mask-border-repeat'> ||

<mask-border-source> =
none |

<mask-border-slice> =
[ <number> | <percentage> ]{1,4} fill?

<mask-border-width> =
[ <length-percentage> | <number> | auto ]{1,4}

<mask-border-outset> =
[ <length> | <number> ]{1,4}

<mask-border-repeat> =
[ stretch | repeat | round | space ]{1,2}

<mask-border-mode> =
luminance |

<image> =
<url> |

<length-percentage> =
<length> |

<url> =
<url()> |

<url()> =
url( <string> <url-modifier>* ) |

<src()> =
src( <string> <url-modifier>* )


Setting a bitmap-based mask border

In this example, we will mask an element's border with a diamond pattern. The source for the mask is a ".png" file of 90 by 90 pixels, with three diamonds going vertically and horizontally:

The image used for the mask examples on this page. The mask is a transparent square with three rows of three diamonds each. The diamonds are a very light, almost white, shade of grey. The middle part between the diamonds is also solid grey. The parts between the outside of the diamonds and the edge of the image are transparent.

To match the size of a single diamond, we will use a value of 90 divided by 3, or 30, for slicing the image into corner and edge regions. A repeat value of round will make the mask slices fit evenly, i.e., without clipping or gaps.


CSS Masking Module Level 1
# the-mask-border

Browser compatibility

BCD tables only load in the browser

See also