https://developer.mozilla.org/en-US/docs/Learn/CSS/Styling_boxes/Borders
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Note: By default, the ninth slice — the center piece — is omitted completely, and the element content appears in the gap that is left. If you want the center of the border image left in, you can do so by including the keyword fill at the end of your border-image-source value, in which case it will stretch to fit the background area.
But fill keyword can be used with border-image and border-image-slice.
