main

/* stylesheet */
.head  {width:100%; height:100px; background:#def;}
.left  {float:left; width:10em; height:400px; background:#ff9;}
.right {float:right; width:10em; height:400px; background:#fbb;}
.main  {width:auto; height:400px; background:#cf9;}
.foot  {clear:both; width:100%; height:100px; background:#def;}

<!--source-->
<div class="head">head</div>
<div class="left">left</div>
<div class="right">right</div>
<div class="main">main</div>
<div class="foot">foot</div>