I'm trying to put two divs side by side. I want the div on the right to take up as much room as it needs (php echos code for an image in this div, so the width needs to be variable for images of different widths) and the one on the left to take up whatever room is left. I can't get it to work without setting the width of the left div in advance, though. Is what I'm asking possible?

<div style="width:400px; float:left; background:#00ee00;"> 
<div style="width:100px; float:right; background:#ee0000;"> 
fixed
</div>
<div style="display:block; margin-left:auto; margin-right: auto; background:#0000ee;"> 
XXXXX
</div>
</div>

If you float an element without a set width it will be as wide as the longest horizontal line of content plus left and right padding.