The border-image-outset property specifies the amount by which the border image area extends beyond the border box.
The CSS border-image-outset property is a shorthand property used to specify the distance by which an element’s border image is placed outside its box.
Tip: Also look at the border-image property (a shorthand property for setting all the border-image-* properties).
border-image-outset: length|number|initial|inherit;
Note: The border-image-outset property can take from one to four
values (top, right, bottom, and left sides). If the fourth value is omitted, it
is the same as the second. If the third one is also omitted, it is the same as
the first. If the second one is also omitted, it is the same as the first.