IDCh,
Нет прописанного свойства - в самом теге, поэтому 1-е нажатие - определяет его отсутствие
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Тег IFRAME</title>
</head>
<body>
<style>
.blockStyle {
position: relative;
visibility: visible;
}
.buttons {
position: relative;
}
</style>
<script type="text/javascript">
function changeVis (id) {
var Th_Obj=document.getElementById(id);
//alert(id)
if (Th_Obj.style.visibility == "visible"){Th_Obj.style.visibility = "hidden"; return; }
else {Th_Obj.style.visibility = "visible"; return; }
}
</script>
<div id="block" style="visibility:visible" class="blockStyle">Number One</div>
<input class="buttons" type="button" value="Button" onClick="changeVis('block')">
</body>
</html>