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

/* <number> value */
mask-border-outset: 1.5;

/* top and bottom | left and right */
mask-border-outset: 1 1.2;

/* top | left and right | bottom */
mask-border-outset: 30px 2 45px;

/* top | right | bottom | left */
mask-border-outset: 7px 12px 14px 5px;

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

The mask-border-outset property may be specified as one, two, three, or four values. Each value is a <length> or <number>. Negative values are invalid.

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

<length>

The size of the mask border outset as a dimension.

<number>

The size of the mask border outset as a multiple of the corresponding border-width.

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

This property doesn't appear to be supported anywhere yet. When it eventually starts to be supported, it will serve to move the mask away from the inner edge of the element's border box — you can use it to make the mask start from part way across the border, rather than the inside of it.

mask-border-outset: 1rem;

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

-webkit-mask-box-image-outset: 1rem;

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