There's not much you can do about the content on the right side. When the window shrinks to be smaller than the width of the content, the content slides offscreen to the right. However, you can make sure your left content doesn't get lost by wrapping everything in a container and setting the min-width
on that parent.
There are a number of possibilities
Option 1: Static widths
Option 2: Semi-fluid widths
Option 3: Completely fluid widths
Code for Option 1:
html
<div id='content'>
<div class='border-content'></div>
<div id="center"></div>
<div class='border-content'></div>
</div>
css
#content {
min-width: 400px;
border: 3px solid #9ef;
}
#content:after {
display: block;
clear: both;
content: '';
}
.border-content {
float: left;
width: 40px;
height: 200px;
background: #eee;
}
#center{
float: left;
width:300px;
height:200px;
background:url(http://placekitten.com/300/200) no-repeat;
}