CSS3 allows web designers to specify multiple background images for box elements, using nothing more than a simple comma-separated list.
Browser support for multiple backgrounds is now relatively widespread with Mozilla Firefox (3.6+), Safari/Chrome (1.0/1.3+), Opera (10.5+) and even Internet Explorer (9.0+) all implementing the feature.
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The only problem is that the sliding doors technique won’t work with transparent pngs on the edges. The center background will show through. But border-image is probably more appropriate for that anyway.
Jeff: I think matt explained it quite well in words. Think of the sliding door example. You have 3 background images: left cap, center fill, right cap. Without using border-image or border-radius, the center fill would fill the whole div, including the areas of the right and left caps that would be rounded, showing the center fill texture instead of a transparency showing thru to the background.