html

<div class=mod clearfix>                
    <div class=ml >
        <div class=modcon>
             <p>左侧自适应</p>
        </div>       
    </div>               
    <div class=mr>
        <p>右侧定宽</p>         
    </div>               
</div>

css

.ml{float:left;width:100%;}
.ml .modcon{margin-right:210px;}
.mr{float:right;width:190px; margin-left:-190px;}