#container{width: 600px}
#content_left, #content_right{float: left;}
#content_left{width: 400px;}
#content_right{width: 200px;}
.clear{clear: both;}

<div id="container">
    <div id="content_left">
    </div>
    <div id="content_right">
    </div>
    <div class="clear"></div>
</div>

#container{overflow:hidden;}  ??????

Source