title: Why do we need multiple background-color rules?
body: <p>I'm a novice in Css and it's difficult to search for this specific case on the internet, so I post a question here.</p>
  
  <p>I'm working on an existing code base and I see something like this.</p>
  
  <pre><code>li {
    background-color: #000 \9;
    background-color: rgba(0, 0, 0, 0);
  }
  </code></pre>
  
  <p>I don't understand the meaning of <code>\9</code>. But it looks to me the rules are duplicates and I should remove one of them.</p>
  
  <p>Could you please explain the <code>\9</code> and should I remove one rule? Thanks.</p>
  
  <p>The reason I'm asking because I'm converting some css files to less files and <code>\9</code> is causing a compilation problem.</p>