per scritto al volo

<html>
<head>
<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script language="JavaScript">
function controlla(){
if(document.prova.campoa.value==''){
alert('Campo a vuto inserire prima valore.')
document.prova.campoa.focus()
}
}
</script>
</head>

<body>
<form name="prova" method="post">
<input type="text" name="campoa">
<input type="text" name="campob" onFocus="return controlla()">
</form>
</body>
</html>


Ciao