Se ho capito cosa vuoi fare:
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Cambio classe</title>
<style type="text/css">
<!--
#menu {
border: 1px dotted #CCCCCC;
width: 200px;
margin-right: auto;
margin-left: auto;
}
#span-0 {
display: block;
height: 20px;
width: 180px;
margin-top: 3px;
margin-right: auto;
margin-left: auto;
}
#span-1 {
display: block;
height: 20px;
width: 180px;
margin-top: 3px;
margin-right: auto;
margin-left: auto;
}
#span-2 {
display: block;
height: 20px;
width: 180px;
margin-top: 3px;
margin-right: auto;
margin-left: auto;
}
.span-attivo{
background-color: #666666;
}
.span{background-color: #CCCCCC;
cursor:pointer;
}
-->
</style>
<script type="text/javascript">
<!--
function cambiaclasse(id,classeon,classeoff){
var span = document.getElementsByTagName('span')
for(var i=0; i<span.length; i++){
if(id!=span[i].id)span[i].className=classeoff
else span[i].className=classeon
}
}
//-->
</script>
</head>
<body>
<div id="menu"><span id="span-0" onclick="cambiaclasse(this.id,'span-attivo','span')" class="span">span 0</span><span id="span-1" class="span" onclick="cambiaclasse(this.id,'span-attivo','span')">span 1</span><span id="span-2" class="span" onclick="cambiaclasse(this.id,'span-attivo','span')">span 2</span></div>
</body>
</html>
P.S. Forse la stessa cosa si può fare anche con le pseudo classi in css, ma non sono sicuro.