SNTF Customer Error Markers 2

<form action="#" method="post" class="formtastic">
  <fieldset class="inputs">
    <ol>    
      <li class="string">
        <label for="first_name">First Name</label>
        <input id="post_first_name" name="post[first_name]" type="text" />
      </li>
      <li class="string">
        <label for="last_name">Last Name</label>
        <input id="post_last_name" name="post[last_name]" />
      </li>
    </ol>
  </fieldset>
</form>