codice:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento senza titolo</title>
<script>
var s=50;
function ini(){
if(s<600){s+=10;
document.getElementById('uno').style.width=s+'px';
setTimeout("ini()",10);}
}
</script>
</head>
<body onload="document.getElementById('uno').style.width=50+'px';">
<div id="uno" style="border:solid 1px #003";>stop a 600px</div>

<input type="button" onclick="ini()" value="start" />
</body>
</html>
ho scritto questo a volo