The basic idea
The border-image shorthand property has 3 parts:
border-image: url(border-image.png) 25% repeat;
Essentially, these allow you to specify:
1. An image to use as the border
2. Where to slice that image, dividing the image into 9 sections
3. How the browser should apply those sections to the edges of your element
The pertinent details
Let’s look at each part of the process in a little more detail. The first part is easy, and is familiar from the background-image property. For demonstration purposes I’ll use this image, which is 100px x 100px:
Slicing your image
The second part can have from one to four values, much like the border-width property, and they are specified in the same order: top, right, bottom, left. You can use percentages or pixels. Strangely, the percentages require the “%”, while pixels should be listed without the “px”:
border-image: url(my-image.gif) 25% 30% 10% 20% repeat;
border-image: url(my-image.gif) 25 30 10 20 repeat;
In this case, since my image is 100px x 100px, the two rules above are equivalent – they slice the image in the same places. I’ve added some dimensions on my image to demonstrate:
Repeat, Round, Stretch
border-image will always place the corner sections of your image into the corresponding corners of your element box, but the third part of the shorthand rule tells the browser how to treat the middle sections of your image — the ones that will go along the edges of your element. Repeat (repeat, or tile, the image) and stretch (stretch, or scale, the image) are pretty self-explanatory. Round means tile the image but only so that a whole number of tiles fit, and otherwise scale the image. Right now, Safari and Chrome interpret round as repeat. There can be up to two values: one for the top and bottom edges of the element, and one for the left and right. Here’s an example with the top/bottom value set to repeat, and the left/right value set to stretch:
#example-one {
border-width:25px 30px 10px 20px;
-moz-border-image:url("border-image.png") 25 30 10 20 repeat stretch;
-webkit-border-image:url("border-image.png") 25 30 10 20 repeat stretch;
border-image:url("border-image.png") 25 30 10 20 repeat stretch;
}
Border-width
border-image won’t do anything if you don’t specify a width for your border. For browsers that understand border-image, your image slices will be scaled to the specified width. If you use the border shorthand property, it provides a nice fallback for browsers that don’t:
#example-two {
border:50px double orange;
-moz-border-image:url("border-image.png") 25 30 10 20 repeat;
-webkit-border-image:url("border-image.png") 25 30 10 20 repeat;
border-image:url("border-image.png") 25 30 10 20 repeat;
}
Or you can specify each width individually (in this example I’ve specified widths such that the image slices aren’t scaled at all):
#example-three {
border-color:orange;
border-style:double;
border-width:25px 30px 10px 20px;
-moz-border-image:url("border-image.png") 25 30 10 20 repeat;
-webkit-border-image:url("border-image.png") 25 30 10 20 repeat;
border-image:url("border-image.png") 25 30 10 20 repeat;
}
Using a plain border at the same widths as your border-image won’t always be ideal, however, so you may want to use conditional stylesheets to give IE some different border styles altogether.
Source : http://css-tricks.com/understanding-border-image/#more-6883
