<html>
<head>
<meta charset="utf8">
<title>Test Popup Overlay</title>
<style type="text/css">
.overlay {
  position: fixed;
  z-index: 1000;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: rgba(0,0,0,0.5);
}
.modal {
  position: fixed;
  z-index: 1001;
  left: 50%;
  top: 50%; 
  width: 200px;
  height: 100px;
  margin-left: -100px;
  margin-top: -50px;
  background-color: #ffffff;
  text-align: center;
  line-height: 100px;
}
</style>
</head>
<body>
<div class="overlay"></div>
<div class="modal">Popup</div>
<div>Some text.</div>
</body>
</html>