title: What is the use of @namespace in CSS?
body: <p>From the <a href="http://www.w3.org/TR/selectors/" rel="noreferrer">doc</a>, I found the use of namespace like below:</p>
  
  <pre><code>@namespace foo url(http://www.example.com);
   foo|h1 { color: blue } 
  </code></pre>
  
  <p>But I want to know more about this. Why do we use it?</p>