Since you have only one fixed width column, float it left and that is it. As for the second column, do not specify float and width; this makes sure it is 100% wide. But you must add a left margin; otherwise the second column will interfere with the floated column e.g.
- aqua background will appear behind blue background (turn off the blue background to see what I mean)
- if second column becomes taller than first one, additional content will start appearing below the first column.
<div id="wrapper">
<div id="left" style="background-color: Blue; height: 100px; float: left; width: 200px;"></div>
<div id="right" style="background-color: Aqua; height: 100px; margin-left: 200px;"></div>
<div style="clear: both;"></div>
</div>
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