クリックで表示、非表示の切り替え

ぴよ
ぴよ
ぴよ
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title></title>
<title></title>
<script type="text/javascript">
<!--
var bv = navigator.appVersion.charAt(0);
var bn = navigator.appName.substr(0,2);
var IE = false;
var NN = false;
var flg = true;
if      ((bv >= 4)&&(bn == "Mi")){IE = true;}
else if ((bv >= 5)&&(bn == "Ne")){NN = true;}
else if ((bv >= 6)&&(bn == "Op")){IE = true;}

function pop_in(){
	if(IE){obj = document.all.item("pop");}
	if(NN){obj = document.getElementById("pop");}
	if(flg){
		obj.style.visibility = "visible";
		flg = false;
	}else{
		obj.style.visibility = "hidden"; 
		flg = true;
	}
}
//-->
</script>
<style type="text/css">
<!--
#pop {
	visibility: hidden;
	width: 400px;
	border : 1px solid #0000ff;
	padding : 5px;
}
-->
</style>
</head>
<body>
<table style="width:400px;">
<tr><td align="right" style="border : 1px solid #ff0000;">
<a href="#" onClick="pop_in();">●</a></td></tr>
</table>
<div id="pop">
ぴよ<br>
ぴよ<br>
ぴよ<br>
</div>
</body>
</html>