html

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

css

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