vedi se questo puō andare bene

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>


<script>

function check() {
    var c_numero = document.getElementById('numero');
    var c_testo = document.getElementById('testo');

    if (isNaN(c_numero.value)) {
        c_numero.style.background = "#cc0000";
        c_numero.focus();    
        return false;
    }
    
    if (c_testo.value.replace(/\s/g, '') == "") {
        c_testo.style.background = "#cc0000";
        c_testo.focus();  
        return false;
    }
}
</script>



<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>

<body>

<form action="" name="modulo" onSubmit="return check();">
<input type="text" name="numero" id="numero" onblur="this.style.background='#fff'">inserire un intero

<input type="text" name="testo" id="testo" onblur="this.style.background='#fff'">campo obbligatorio 
<input type="submit" value="OK"> 
</form>
</body>
</html>