The border-image-sliceCSSproperty specifies inward offsets from the top, right, bottom, and left edges of the image, dividing it into nine regions: four corners, four edges and a middle.
The middle is discarded (treated as fully transparent) unless the fill keyword is present.
An integer or zero or more decimal digits followed by a dot (.) followed by one or more decimal digits. The first character may be preceded by a sign (- or +). The last character may be succeeded by an exponent (e or E) and an integer.
<percentage>
Specifies the percentage using a number followed by a percent sign (%).
fill
Causes the middle part of the border-image to be preserved.