title: are the following css styles are valid
body: <p>I am reading a Responsive website book, where I see the styles definition like</p>
  
  <pre><code>.main-navigation 
    {
      li {position: relative;}
      .children 
            {
              left: -999em;
              position:absolute;
            }
     }
  </code></pre>
  
  <p>I have also tried in my css file, but seems not to be working. 
  Now my question :) Can we define css style in this way?</p>