how to put one div over another

HTML
<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Web page</title>
  </head>
  <body>
    <div id="overlay">
    </div>
    <div id="originalDiv">
    </div>
  </body>
  <style>
    #overlay {
      width: 100px;
      height: 100px;
      background-color: red;
      z-index: -1;
      position:absolute;
      top:50px;
      left:50px;
    }
    
    #originalDiv {
      width: 100px;
      height: 100px;
      background-color: blue;
      z-index: 1;
      position:absolute;
      top:0px;
      left:0px;
    }
  </style>
</html>
Source