title: How to cut a part of div
body: <p>I try to give you a broad picture of my problem. Suppose I have a overlay <code>div</code> on my page with <code>z-index: 99;</code>, with <code>background-color: rgba(0,0,0,0.5)</code>. Then I want to remove a part of overlay <code>div</code>, for example <code>100 x 100px</code> in <code>top: 50px;</code> and <code>left: 200px;</code>.</p>
  
  <p>By removing I mean that exclude a part of overlay <code>div</code> to make that part visible and remove the <code>background-color</code> from that.</p>
  
  <p>How can I do that?</p>