`mask-border-width` CSS property

Baseline Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

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

Syntax

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

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

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

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

/* top and bottom | left and right */
mask-border-width: 2em 3em;

/* top | left and right | 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: revert;
mask-border-width: revert-layer;
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).

Values

<length-percentage>: 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.
<number>: The width of the mask border, specified as a multiple of the corresponding border-width. Must not be negative.
auto: 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 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
Computed value	as specified, but with relative lengths converted into absolute lengths
Animation type	discrete

Formal syntax

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

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

Examples

Basic usage

mask-border-width defines the width of the border mask — setting this to a different value to mask-border-slice will cause the slices to be scaled to fit the border mask.

/* Final mask scaled down to 20px wide */
mask-border-slice: 30 fill;
mask-border-width: 20px;

Chromium-based browsers support an outdated version of this property — mask-box-image-width — with a prefix:

-webkit-mask-box-image-width: 20px;

Note: The mask-border page features a working example (using the out-of-date prefixed border mask properties supported in Chromium), so you can get an idea of the effect.

Specifications

Specification
CSS Masking Module Level 1 # the-mask-border-width

Browser compatibility

	Desktop					Mobile
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on iOS	Samsung Internet	WebView Android	WebView on iOS
`mask-border-width`	1	79	No	15	17.2 3.1	18	No	14	17.2 2	1.0	4.4	17.2 2