Un id è unico.
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<style type="text/css">
<!--
.divHidden  {visibility:hidden; background-color:#208FFF;}
.divVisible {visibility:visible; background-color:#208FFF;cursor:hand}
-->
</style>
<script language="JavaScript" type="text/javascript">
<!--
function hideDiv() {
 var listElement = document.all;
 for (i=0;i<listElement.length;i++) {
  var anObj = listElement.item(i);
	if (anObj.tagName == "DIV") {
	 if (anObj.className == "divVisible") {
	  anObj.className = "divHidden";
	 } else {
	  anObj.className = "divVisible";
	 } // if (anObj.className == "divVisible")
	} // if (anObj.tagName == "DIV"
 } // for (i=0;i<listElement.length;i++)
} // function hideDiv()
//-->
</script>


</head>
<body>
<div id="ita1" class="divVisible" style="background-color:yellow">salut mon gars</div>
<div id="ita2" class="divVisible" style="background-color:blue">salut mon gars</div>
<div id="ita3" class="divVisible" style="background-color:green">salut mon gars</div>
<div id="ita4" class="divVisible" style="background-color:pink">salut mon gars</div>
<input type="button" value="Clicca" onclick="hideDiv();">

</body>
</html>