Create four images for your corners. Most graphics programs have a tool that will create rounded-off squares. I’ll be using this square here…

…and I’m going to cut off the corners to get my four images:

In the spot where I want the box to show up, I create a container div to hold the box, a div for the top row and a div for the bottom row. Between the top and bottom rows, I add my content. In the top and bottom row divs, I add the left corner image and set the inline style to read display: none;. This makes the image invisible unless I make it visible through the stylesheet. That way, I can hide the effect from incompatible browsers by not showing them the stylesheet.

<div class="roundcont">
 
<div class="roundtop">
 
<img src="tl.gif" alt="" 
 
width="15" height="15" class="corner" 
 
style="display: none" />
 
 
</div>
 
 
<p>Lorem ipsum dolor sit amet, consectetur adipisicing 
 
elit, sed do eiusmod tempor incididunt ut labore et 
 
dolore magna aliqua. Ut enim ad minim veniam, quis 
 
nostrud exercitation ullamco laboris nisi ut aliquip 
 
ex ea commodo consequat. Duis aute irure dolor in 
 
reprehenderit in voluptate velit esse cillum dolore eu 
 
fugiat nulla pariatur. Excepteur sint occaecat cupidatat 
 
non proident, sunt in culpa qui officia deserunt mollit 
 
anim id est laborum.</p>
 
 
 
<div class="roundbottom">
 
<img src="bl.gif" alt="" 
 
width="15" height="15" class="corner" 
 
style="display: none" />
 
 
</div>
</div>
 

The CSS sets the width and background color of the container object and the color of the text inside. The margins on interior paragraphs are set so that the text doesn’t sit right up against the edge of the box.

Then the top and bottom divs are given a background image that contains the right corner images and the left corner images from the HTML code are enabled.

.roundcont {
 
width: 250px;
 
background-color: #f90;
 
color: #fff;
}
 
.roundcont p {
 
margin: 0 10px;
}
 
.roundtop { 
 
background: url(tr.gif) no-repeat top right; 
}
 
.roundbottom {
 
background: url(br.gif) no-repeat top right; 
}
 
img.corner {
 
width: 15px;
 
height: 15px;
 
border: none;
 
display: block !important;
}

Here’s how it looks when it’s all put together.

Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.

This works in IE6, Mozilla 1.3, and Opera 7 on Windows. It should work in most other modern browsers as well.

Update: Julian Bond asked if this will work with the container div set to 100% width. Here’s the box again. The only thing changed is the width of the roundcont class.

Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.