Purpose:
Enhance CSS with styled lines between layout items in grid, flex, masonry, and multi-column containers (same as column-rule in multicol).
New Properties & Behavior:
row-rule, column-rule to define width, style, and color of decorations.
Complex patterns with repeat().
gap-rule-break to control where decorations start/end.
gap-rule-paint-order to define overlap order between row and column.
gap-rule-outset to adjust start/end positions.