gistfile1.html

<form is="ajax-form" action="test" method="post">

  <paper-radio-group id="wifi" selected="always">
    <paper-radio-button name="always" label="Always"></paper-radio-button>
    <paper-radio-button name="pluggedin" label="Only when plugged in"></paper-radio-button>
    <paper-radio-button name="never" label="Never"></paper-radio-button>
  </paper-radio-group>

  <input type="submit">

</form>

<script>

  var wifi = document.getElementById('wifi');
  document.querySelector('form').addEventListener('submitting', function(e) {
    var formData = e.detail.formData;
    if (!wifi.selected) {
      console.log('validation error')
    } else {
      formData[wifi.id] = wifi.selected;
    }
  });

</script>