Exemple de code du sélecteur CSS :required.

<style type="text/css">
 .defaut{ margin:5px }
 :required{ 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>