Created attachment 463732 [details]
testcase

If no -webkit-mask-box-image-slice is provided, then https://searchfox.org/wubkat/rev/70230d10f272f17151f1e77d496553716b449cea/Source/WebCore/rendering/style/NinePieceImage.h uses

    data.imageSlices = LengthBox(0);

However, if you specify -webkit-mask-box-image-slice:initial, then https://searchfox.org/wubkat/rev/70230d10f272f17151f1e77d496553716b449cea/Source/WebCore/style/StyleBuilderCustom.h#536

    image.setImageSlices(LengthBox());

These are different:
 - LengthBox() has lengths of LengthType::Auto and in NinePieceImage::computeSlices they will behave like Length(100, LengthType::Percent).
 - LengthBox(0) has Length(0, LengthType::Fixed).

Using LengthBox() seems clearly wrong, I think there are 2 possibilities:
 - Switch both places to using LengthBox(0).
   This will match the current behavior when no -webkit-mask-box-image-slice is provided.
   This will obey https://www.w3.org/TR/2014/WD-css-masking-1-20140213/#the-mask-box-slice and https://drafts.fxtf.org/css-masking-1/#the-mask-border-slice
 - Switch both places to using LengthBox(Length(100, LengthType::Percent), Length(100, LengthType::Percent), Length(100, LengthType::Percent), Length(100, LengthType::Percent))
   This will match the current behavior when using -webkit-mask-box-image-slice:initial
   This will make it consistent with border-image-slice