ie5下，appendChild方法加的iframe操作出错，说是“类不支持automation操作”？

<HTML>
<HEAD>
<META NAME="GENERATOR" Content="Microsoft Visual Studio 6.0">
<TITLE></TITLE>
</HEAD>
<script language=javascript>
function yyy()
{
var xx=open("about:blank",'xxx')
alert(xx.document.documentElement.outerHTML)
xx.document.write("<script>alert('how are you')</")
xx.document.write("script>")
xx.document.write("<body>alert('how are you')</body>")
alert(xx.document.documentElement.outerHTML)
}
function zzz()
{
oFrame=document.createElement("iframe");
oFrame.src="a.htm"
oFrame.width=100;
oFrame.height=100;
ttt.appendChild(oFrame);
}
</script>
<BODY onselectstart="return false" onclick="zzz()">
sdfasdfasdf
<iframe name=xxx src="b.htm" height=100 width=400></iframe>
<form target=xxx action="javascript:document.write('xx')" id=form1 name=form1>
<input value="xxx" name=my>
<center id=ttt></center>
<input type=submit value="submit" id=submit1 name=submit1>
</form>
</BODY>
</HTML>没有出错呀..我的也是ie5