mask-border-slice

/* All sides */
mask-border-slice: 30%;

/* top and bottom | left and right */
mask-border-slice: 10% 30%;

/* top | left and right | bottom */
mask-border-slice: 30 30% 45;

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

/* Using the `fill` keyword */
mask-border-slice: 10% fill 7 12;

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

The mask-border-slice property may be specified using one to four <number-percentage> values to represent the position of each image slice. Negative values are invalid; values greater than their corresponding dimension are clamped to 100%.

• When one position is specified, it creates all four slices at the same distance from their respective sides.
• When two positions are specified, the first value creates slices measured from the top and bottom, the second creates slices measured from the left and right.
• When three positions are specified, the first value creates a slice measured from the top, the second creates slices measured from the left and right, the third creates a slice measured from the bottom.
• When four positions are specified, they create slices measured from the top, right, bottom, and left in that order (clockwise).

The optional fill value, if used, can be placed anywhere in the declaration.

<number>

Represents an edge offset in pixels for raster images and coordinates for vector images. For vector images, the number is relative to the element's size, not the size of the source image, so percentages are generally preferable in these cases.

<percentage>

Represents an edge offset as a percentage of the source image's size: the width of the image for horizontal offsets, the height for vertical offsets.

fill

Preserves the middle image region. Its width and height are sized to match the top and left image regions, respectively.

The slicing process creates nine regions in total: four corners, four edges, and a middle region. Four slice lines, set a given distance from their respective sides, control the size of the regions.

The above diagram illustrates the location of each region.

• Zones 1-4 are corner regions. Each one is used a single time to form the corners of the final border image.
• Zones 5-8 are edge regions. These are repeated, scaled, or otherwise modified in the final border image to match the dimensions of the element.
• Zone 9 is the middle region. It is discarded by default, but is used like a background image if the keyword fill is set.

The mask-border-repeat, mask-border-width, and mask-border-outset properties determine how these regions are used to form the final mask border.

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

This property doesn't appear to be supported anywhere yet. When it eventually starts to be supported, it will serve to define the size of the slices taken from the source image, and used to create the border mask.

mask-border-slice: 30 fill;

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

-webkit-mask-box-image-slice: 30 fill;

• mask-border
• mask-border-mode
• mask-border-outset
• mask-border-repeat
• mask-border-source
• mask-border-width
• Illustrated description of the 1-to-4-value syntax