title: '[hidden]' selectors in css
body: <p>When we view normalize.css <a href="http://necolas.github.io/normalize.css/" rel="nofollow">http://necolas.github.io/normalize.css/</a> source code, we can see the following:</p>
  
  <pre><code>[hidden], template {
      display: none;
  }
  </code></pre>
  
  <p>What is the meaning of [hidden] ?</p>