CSS标准系列解决背景不能延伸的问题

<div ="content">
<div id="main">
<div id="maincol"></div>
<div id="xcol"></div>
</div>
<div id="subcol"></div>
</div>

#content{width:700px; background:url(img/bg.png)repeat-y;}
#subcol {float:left; width:200px;}
#main {width:500px; float:right;}
#maincol {width:300px;float:left;}
#xcol {width:200px; float:right;}