If you have some regular content, and not only one line of text, the only possible reason I know is to calculate margin.
Here is an example:
HTML
<div id="supercontainer">
<div id="middlecontainer">
<div class="common" id="first">first</div>
<div id="container">
<div class="common" id="second">second</div>
<div class="common" id="third">third</div>
</div>
</div>
</div>
CSS
body {
margin: 0;
padding: 0;
}
.common {
border: 1px solid black;
}
#supercontainer {
width: 1200px;
background: aqua;
float: left;
}
#middlecontainer {
float: left;
width: 104px;
margin: 0 549px;
}
#container {
float: left;
}
#first {
background: red;
height: 102px;
width: 50px;
float: left;
}
#second {
background: green;
height: 50px;
width: 50px;
}
#third {
background: yellow;
height: 50px;
width: 50px;
}
So, #supercontainer is your "whole page" and its width is 1200px.
#middlecontainer is div with content of your site; it's width 102px. In case the width of content is known, you need to divide the page's size to 2, and subtract half of content's width from the result:
1200 / 2 - (102 / 2) = 549;
Yes, I'm also seeing that this is der grosse fail of CSS.
