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;}