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This chapter is from the book

Box Clever: border-image

CSS3 provides you with more powerful properties to control borders. The most interesting of these properties is border-image, which in effect allows you to divide an image into different slices and dress the edges around any box you’d like. This sounds a bit complicated, so let’s walk through a simple example. To demonstrate border-image, I first created a simple image to grab a border from (Figure 4.33).

Figure 4.33

Figure 4.33 A sample border image.

Next, I applied the border image neatly to the border of a larger box in a flexible manner (Figure 4.34). Check out the border-image.html file in the chapter4 code download folder. So, how do you do that?

Figure 4.34

Figure 4.34 The image has been carefully dressed around the example container. Cool!

Try increasing and decreasing the width of the browser, and you should see the border flexibly adjust. The following lines of code are doing the heavy lifting here (border-image is currently supported across all major browsers using vendor prefixes, but I’ve omitted them here for brevity):

border-image: url(border.png) 30 30 30 30 round;
border-width: 30px;

As you’d expect, the url() syntax points to the image you want to use for the border.

The four numbers after that specify the widths of the border slices (be mindful that these are numbers of pixels, even though they have no units) you want to chop the image into. In order, they refer to the top slice, the right slice, the bottom slice, and the left slice, as indicated in Figure 4.35.

Figure 4.35

Figure 4.35 Slicing up the image samurai style.

You can set the slices to any size you like, but it obviously makes sense to divide the image so the slices in the border contain the parts of the image you want. As logic would suggest, because in this case all four slices are the same size, you could write the border-image line using two values or one value, like so:

border-image: url(border.png) 30 round;

After slicing the image, it is applied to the borders of your container: The four corners remain the same. The four sides all tile in one dimension to fill up the borders, however long they are (although see the end of this section for browser differences). The center of the image tiles in both directions to fill up the remaining background space.

How do you control the manner in which the tiling of the sides is handled? You use the round keyword. round specifies that the browser should always show a whole number of repeated border segments and no incomplete fragments, adjusting the size of each segment to maintain the desired result. If you try increasing or decreasing the browser window width in Opera, Firefox, and IE, you’ll see that the size of the balls adjust slightly. Unfortunately, WebKit-based browsers treat round the same as repeat, another value that just tiles the side slices until they fill up each side without rounding. You are therefore left with fragments at each end of the sides, which don’t look great (Figure 4.36).

Figure 4.36

Figure 4.36 border-image fragments look shoddy.

Fragments might look OK as long as you plan the shape of your slices carefully and make sure they don’t differ much in height along their course. Another value to be aware of is stretch, which is the default. If you swap round for stretch in this example or omit it altogether, you’ll get the result shown in Figure 4.37.

Figure 4.37

Figure 4.37 stretch is OK for very small variations in container size but looks terrible if your containers greatly differ in size from the original image.

Again, note that I’ve specified one value here for the repeating behavior of all four sides. If you want to specify different behavior for different sides, you could use two or four sides as logic would suggest; for example, round stretch or round stretch round stretch.

And there’s one more detail you should know about the basic syntax. To actually provide space for your border image, you need to specify a border width, which is why border-width: 30px; was included in the code example. If you don’t do this, you won’t see anything. The border-width property offers additional interesting possibilities: If you make the border width bigger or smaller than the slices within the border-image property, the slice size will scale up or down to suit. So, if you make the border half as big, like so:

border-width: 15px;

the border image slices will be displayed in half dimensions, as shown in Figure 4.38!

Figure 4.38

Figure 4.38 Border images at half the size—very funkalistic.

Border-Image Problems

Using border-image is not all plain sailing, of course, as with most things in web design. Aside from the fact that WebKit-based browsers currently don’t use the round value properly, there are a couple of other issues to keep in mind.

First, you need to determine if you want your central slice to be discarded or not. You might want the central slice to act as the background for your container content, or you might just want to put the border image only in the border. Unfortunately, the default behavior according to the spec is to discard it, but most browsers do the opposite, except for Chrome (try comparing border-image-2.html in Chrome and Opera). You are supposed to be able to control this behavior with border-image-slice, but this property is currently not supported in any browser, so your only option is to fill in your background explicitly with a background-color or repeated background-image, and try to make it match the border-image.

Second, older versions of IE do not support border-image, so you should test to make sure your content is still readable and looks OK in these older IE browsers.

With these two issues in mind, this border-image won’t work at all in old versions of IE, and it looks dreadful in Chrome (Figure 4.39).

Figure 4.39

Figure 4.39 The border is definitely eye-catching, but not in a good way.

To fix these issues, I added a repeated background image to my containers, which is basically just the center slice of the border image with the rough edges removed. Initially, this looked ghastly because the background image extended into the border. But I sorted out the problem with a nifty little property called background-clip. The code additions are as follows:

background: url(../images/parchment-bg.jpg);
background-clip: padding-box;

This snippet of code made the background look better again in Chrome, although it wasn’t perfect, and it made the content readable in Internet Explorer’s decrepit old ancestors. This example is difficult because I am using a complicated background image on a variable width container. Getting a seamless look would be a lot easier to do in a simpler, fixed-width case!

These limitations mean that border-image usage will be slightly more restricted than is ideal for the moment, but it is already usable to great effect in many situations.

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