The border-image-outset property is used to define how far the border image area extends past the border box.
It is a shorthand property of the following:
border-image-source
border-image-slice
border-image-width
border-image-outset
border-image-repeat
border-image-outset: length|number|initial|inherit;
length : Its default value is 0. It is used to specify the distance of the border from the edges.
number: It is used to represent a multiple of the border-width.
initial: It is used to set the property to the default value.
inherit: It is used to inherit the property from the parent element.