:master: io non ho controllato e non so se vada bene

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>
    <title>Pagina senza titolo</title>

    <script language='JavaScript' type='text/javascript'>
function checkForm() {
    var el = document.forms[0].elements;
    for(var i = 0 ; i < el.length ; ++i) {
        if(el[i].type == "radio") 
        {
            var radiogroup = el[el[i].name];
            var itemchecked = false;
            var n = radiogroup.length;
            if(n != undefined)
            {
                for(var j = 0 ; j < radiogroup.length; ++j) {
                    if(radiogroup[j].checked){
                        itemchecked = true;
                        break;
                    }
                    
                }
            }
            else
	        {
                if(radiogroup.checked)
                {
                    itemchecked = true;
                    //break;
                }
        		
	        }
            
            
            if(!itemchecked) {
                alert("Scegli un valore");
                return false;
            }
        }
    }
    return true;
} 

</script>

</head>
<body>
    <form name="myform" action="una pagina da mettere">
        


            <input type='radio' name='R1'></p>
        <input type='submit' value='Visualizza' name='B1' onclick='return checkForm()'>
    </form>
</body>
</html>