/* 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).

<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.

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

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

This property doesn't appear to be supported anywhere yet. When it eventually starts to be supported, it will serve to define 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.

mask-border-width: 30 fill;

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

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

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