codice:
function verifica() {
  if ((document.logon.username.value.length<1) || (isNaN(document.logon.username.value))){
    window.alert("Inserisci l'username!");
    return false; 
  }

  if (document.logon.password.value.length<1) || (isNaN(document.logon.password.value))){
    window.alert("Inserisci la password!");
    return false;
  }    
}
ciao