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

Constituent properties

This property is a shorthand for the following CSS properties:

• mask-border-mode
• mask-border-outset
• mask-border-repeat
• mask-border-slice
• mask-border-source
• mask-border-width

Syntax

/* 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;

Values

<'mask-border-source'>

The source image. See mask-border-source.

<'mask-border-slice'>

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

<'mask-border-width'>

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

<'mask-border-outset'>

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

<'mask-border-repeat'>

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.

<'mask-border-mode'>

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

Formal definition

Formal syntax

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

<mask-border-source> = 
  none     |
  <image>  

<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  |
  alpha      

<image> = 
  <url>       |
  <gradient>  

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

<url> = 
  <url()>  |
  <src()>  

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

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

Examples

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:

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.

<div class="masked">
  This element is surrounded by a bitmap-based mask border! Pretty neat, isn't
  it?
</div>

.masked {
  width: 200px;
  background-color: lavender;
  border: 18px solid salmon;
  padding: 10px;

  -webkit-mask-box-image: url("https://mdn.github.io/shared-assets/images/examples/mask-border-diamonds.png")
    30 fill /          /* slice */
    20px /             /* width */
    1px                /* outset */
    round;             /* repeat */

  mask-border:
    url("https://mdn.github.io/shared-assets/images/examples/mask-border-diamonds.png")
    30 fill /        /* slice */
    20px /           /* width */
    1px              /* outset */
    round;           /* repeat */
}

Specifications

Browser compatibility

See also

• mask-border-mode
• mask-border-outset
• mask-border-repeat
• mask-border-source
• mask-border-width