Exemple de code du sélecteur CSS :optional.

<style type="text/css">
 .defaut{ margin:5px; }
 :optional{ outline:red dotted 2px }
</style>
<form action="?">
  <div class="defaut">
    <b>sans attribut required : </b><br>
    <input type="text" value=""> 
    <input type="checkbox"> 
    <input type="radio">
    <select><option>required</option><option>required</option></select><br><br>
    <textarea></textarea> 
  </div>
  <div class="defaut">
      <b>avec attribut required : </b><br>
      <input type="text" required value="">
      <input type="checkbox" required>
      <input type="radio" required >
      <select required><option>required</option><option>required</option></select><br><br>
      <textarea required></textarea>
  </div>
</form>