title: Is a concave border radius possible?
body: <p>Here is a simple convex example.</p>
  
  <p><a href="http://jsfiddle.net/swY5k/" rel="noreferrer">http://jsfiddle.net/swY5k/</a></p>
  
  <pre><code>#test{
      width: 200px;
      height: 200px;
      background: #888888;
      border-radius: 50px;
  }
  </code></pre>
  
  <p>However, I want a concave border radius.</p>
  
  <p>I tried making the border-radius negative but this did not work.</p>
  
  <p>Example of concave/convex:</p>
  
  <p><img src="https://i.stack.imgur.com/kJnVM.gif" alt="enter image description here"></p>