no no,hai ragione,sono un cretino,a me interessano i tag TD :adhone:

facendo così funziona tutto!
Thanks a lot Reale_Augello ALIAS var...ALIAS...


<html>
<head>
<title>Prova</title>

<script language="JavaScript">

function MyBlink() {
var ref=document.getElementsByTagName('td');
for (i=0; i<ref.length; i++)
if (ref[i].id.indexOf('cella')!=-1) {
ref[i].className=="c1" ? ref[i].className="c2" : ref[i].className="c1";
}
window.setTimeout("MyBlink()",1000);
}

</script>

<style type="text/css">
.c1{BACKGROUND-COLOR:blue;}
.c2{BACKGROUND-COLOR:red;}
</style>

</head>

<body onLoad="MyBlink();">
<table>
<tr><td class="c1" id="cella1">a</td></tr>
<tr><td class="c1" id="cella2">b</td></tr>
</table>

</body>
</html>