Implementation
  @override
List<BorderSide>? get borderSides {
  BorderSide? leftSide = CSSBorderSide._getBorderSide(this, CSSBorderSide.LEFT);
  BorderSide? topSide = CSSBorderSide._getBorderSide(this, CSSBorderSide.TOP);
  BorderSide? rightSide = CSSBorderSide._getBorderSide(this, CSSBorderSide.RIGHT);
  BorderSide? bottomSide = CSSBorderSide._getBorderSide(this, CSSBorderSide.BOTTOM);
  bool hasBorder = leftSide != null || topSide != null || rightSide != null || bottomSide != null;
  return hasBorder
      ? [
          leftSide ?? CSSBorderSide.none,
          topSide ?? CSSBorderSide.none,
          rightSide ?? CSSBorderSide.none,
          bottomSide ?? CSSBorderSide.none
        ]
      : null;
}