[css3-background] border-image-slice %age values

Currently implementing border-image in BlueGriffon, I have a question
relative to border-image-slice. I don't understand the use case for
percentages inside border-image-slice values. They also drastically
complexify the implementation of a wysiwyg UI for this property.

It seems to me that pixels (as <number> in the spec) are enough
since this property is likely to be always applied to bitmap images
easily controllable at pixel-level by the web author, or vectorial
images with a default viewport setting giving it again a bitmap
representation.

I understand %ages could be useful to use SVG images entirely drawn
using #ages as border images but the resulting complexity in wysiwyg
editors seems to me overkill.

</Daniel>