title: style span when focused on form input
body: <p>Small question but, cannot find method to solve this little problem. I have html form</p>
  
  <pre><code> &lt;div id="todolist"&gt;
        &lt;span class="add-on"&gt;OK&lt;/span&gt;
        &lt;input class="addtodo" placeholder="New todo task" name="TITLE"  type="text" &gt;
   &lt;/div&gt;
  </code></pre>
  
  <p>in CSS</p>
  
  <pre><code> #post-todo span{   
       color:#aaa;
   }
  </code></pre>
  
  <p>I want to change <code>color:#333</code> when focused on <code>input</code>,
  how do this?</p>