angular VALIDATION errors don't appear when I submit code example

Example: angularjs show form validation errors

<ul>
  <li ng-repeat="(key, errors) in form.$error track by $index"> <strong>{{ key }}</strong> errors
    <ul>
      <li ng-repeat="e in errors">{{ e.$name }} has an error: <strong>{{ key }}</strong>.</li>
    </ul>
  </li>
</ul>