Note: this post has been moved from the yosle.com blog, which I am in the process of taking down.

Round corners. Something that should be easy, right? Not so much. It seems they were left out of the CSS (less than 3) specification. No matter, there are a couple of ways to hack them out and make them look good. Basically there are two techniques: layering a few empty tags, and using images. There are some ways to do it with javascript as well, but I wont cover them because javascript is unnecessary. The layering technique is the most concise and clean, IMO, so that’s what this post will focus on.

Err… No images? No javascript?

The line-layering technique uses no images and no javascript, only display: block bold tags layered on top of each other. We simply create lines with decreasing side margins, stick them on top of each other and we have a well-emulated round corner. First I’ll show, then explain.

And the code

.b1f, .b2f, .b3f, .b4f{font-size:1px; overflow:hidden; display:block;}
.b1f {height:1px; background:#ddd; margin:0 5px;}
.b2f {height:1px; background:#ddd; margin:0 3px;}
.b3f {height:1px; background:#ddd; margin:0 2px;}
.b4f {height:2px; background:#ddd; margin:0 1px;}
.contentf {background: #ddd;}
.contentf div {margin-left: 5px;}

<b class="b1f"></b><b class="b2f"></b><b class="b3f"></b><b class="b4f"></b>
    <div class="contentf">
        <div>Round FILL!!</div>
    </div>
<b class="b4f"></b><b class="b3f"></b><b class="b2f"></b><b class="b1f"></b>

You notice that there are four b#f definitions. Each one represents a single 1px tall line. Element b1f is shorter than all the other lines, b2f is a little longer, b3f is longer still, and b4f is almost the size of the main content element. This is easy to see if we change the color of each line.

.b1f, .b2f, .b3f, .b4f{font-size:1px; overflow:hidden; display:block;}
.b1f {height:1px; background:#F00; margin:0 5px;}
.b2f {height:1px; background:#0F0; margin:0 3px;}
.b3f {height:1px; background:#00F; margin:0 2px;}
.b4f {height:2px; background:#F0F; margin:0 1px;}
.contentf {background: #ddd;}
.contentf div {margin-left: 5px;}

Now that we have the basics down, this technique is easy to extend. Maybe you want a block with a border. Thats pretty easy too. We just add 1 pixel side borders to the b2f, b3f, b4f, and content definitions.

.b1, .b2, .b3, .b4{font-size:1px; overflow:hidden; display:block;}
.b1 {height:1px; background:#888; margin:0 5px;}
.b2 {height:1px; background:#ddd; border-right:2px solid #888; border-left:2px solid #888; margin:0 3px;}
.b3 {height:1px; background:#ddd; border-right:1px solid #888; border-left:1px solid #888; margin:0 2px;}
.b4 {height:2px; background:#ddd; border-right:1px solid #888; border-left:1px solid #888; margin:0 1px;}
.contentb {background: #ddd; border-right:1px solid #888; border-left:1px solid #888;}
.contentb div {margin-left: 5px;}

<b class="b1"></b><b class="b2"></b><b class="b3"></b><b class="b4"></b>
    <div class="contentb">
        <div>Round Border!!</div>
    </div>
<b class="b4"></b><b class="b3"></b><b class="b2"></b><b class="b1"></b>

Can we go further? I think so. It could be that you want a box with a special color for the header. Well, it turns out thats not very difficult, either.

.b1h, .b2h, .b3h, .b4h, .b2bh, .b3bh, .b4bh{font-size:1px; overflow:hidden; display:block;}
.b1h {height:1px; background:#aaa; margin:0 5px;}
.b2h, .b2bh {height:1px; background:#aaa; border-right:2px solid #aaa; border-left:2px solid #aaa; margin:0 3px;}
.b3h, .b3bh {height:1px; background:#aaa; border-right:1px solid #aaa; border-left:1px solid #aaa; margin:0 2px;}
.b4h, .b4bh {height:2px; background:#aaa; border-right:1px solid #aaa; border-left:1px solid #aaa; margin:0 1px;}
.b2bh, .b3bh, .b4bh {background: #ddd;}
.headh {background: #aaa; border-right:1px solid #aaa; border-left:1px solid #aaa;}
.headh h3 {margin: 0px 10px 0px 10px; padding-bottom: 3px;}
.contenth {background: #ddd; border-right:1px solid #aaa; border-left:1px solid #aaa;}
.contenth div {margin-left: 12px; padding-top: 5px;}

<b class="b1h"></b><b class="b2h"></b><b class="b3h"></b><b class="b4h"></b>
    <div class="headh">
        <h3>Here is your Header!</h3>
    </div>
    <div class="contenth">
        <div>Look ma, no images!</div>
    </div>
<b class="b4bh"></b><b class="b3bh"></b><b class="b2bh"></b><b class="b1h"></b>

There are some drawbacks to this technique, however. If you want a box with big radii on the corners, you will be creating a lot of bold elements for each box. As the radius gets bigger, it also becomes harder to figure out the margin for each line. Another drawback is the ’sharpness’ of the corners. Images will produce smoother, anti-aliased corners. If the corner radius is small, its hardly noticeable, though.

Images

Images are the more versatile alternative. They are a bit more difficult to use and are much more clunky code-wise unless your boxes are a fixed size. The general approach for variable size boxes using my favorite technique, the thrashbox, is about as follows: make an over-sized image of the complete box, anchor one corner of the image, then overlay the other three corners in their respective places.

No sense in explaining it in depth as its been hashed out many times. I will, however, point you to the most awesome site regarding the technique. The site is called SpiffyBox. Not only does it show you how to do it, it has a little app that creates images for you based on your criteria! Cooool. The image creator was the selling point for me.

Another image technique is to use four separate images. This post outlines how its done very nicely.

CSS3

There is a border-radius property in CSS3. Check out this post for more information.