type radio obbligatorio

[br1]

<script>
function chk(f) {
  r = f.NomeRadio;
  rv = false;
  for(var i=0;i<r.length;i++) {rv=rv||r[i].checked};
  return rv;
}
</script>
<form onsubmit="return chk(this)">
<input type=radio name="NomeRadio" value="1">
<input type=radio name="NomeRadio" value="2">
<input type=radio name="NomeRadio" value="3">
<input type=radio name="NomeRadio" value="4">
<input type=submit>
</form>