#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
