title: nested css rules
body: <p>Some time ago I saw an example of a css file, where the css rules/selectors where specified in a nested way, e.g. something like this:</p>
  
  <pre><code>div.a {
    color: red;
    div.b {
      font-weight: bold;
    }
  }
  </code></pre>
  
  <p>I'm not sure where I saw that (probably in a SO question), or whether it was exactly as shown above.</p>
  
  <p>My questions: Is the above CSS correct/valid? Is it a standard way to specify CSS rules or is this a browser-dependent hack?</p>