More author's comments:

I am proposing following attributes:

  background-image-margin-top: length;
  background-image-margin-bottom: length;
  background-image-margin-left: length;
  background-image-margin-right: length;

with additional:
  background-image-fill-policy: [middle-stretch] [top-stretch]
[bottom-stretch] [left-stretch] [right-stretch];

Instead of all proposed border-image-* and new background-image-*
attributes.

Some thoughts about existing CSS3 proposal:

To render box with borders and background will take 10 images to download.
This feature will be quite popular,
but each http: connection takes near 40 kbytes of machine instructions just
to establish and serve connection (without transfer). To be short it is
better to reduce pollution.

Artistic borders being designed will be used in 99% with artistic (image)
backgrounds. I think it is better to keep such borders and background design
in one image.

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Other possible way how to implement this is to use separate image definition
file beside the image itself.
Instead of adding attributes into CSS element to use just background-image:
url(image.png/image.def).

Thanks.

Andrew Fedoniouk.
http://terrainformatica.com