iframe奇怪的问题

Question

look
-----------------------------------------------
<style>
.hide{
display:none;
}
</style>
<button onclick="show('f1');">f1</button>
<button onclick="show('f2');">f2</button>
<button onclick="show('f3');">f3</button><script type="text/javascript">
function show(obj){
var frms = document.getElementById("container").childNodes;
for(var i=0;i<frms.length;i++){
if(frms[i].id==obj)
frms[i].className="";
else
frms[i].className="hide";
}
}
</script>
<div id="container">
<iframe id="f1" src="http://community.csdn.net/Expert/topic/5318/5318127.xml?temp=.7332117" width="100%" height="500px"></iframe>
<iframe id="f2" class="hide" src="http://community.csdn.net/Expert/topic/5318/5318051.xml?temp=.2589075" width="100%" height="500px"></iframe>
<iframe id="f3" class="hide" src="http://community.csdn.net/Expert/topic/5317/5317659.xml?temp=.2592127" width="100%" height="500px"></iframe>
</div>