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<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
#overlay {
  position: fixed;
  display: none;
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: rgba(0,0,0,0.5);
  z-index: 2;
  cursor: pointer;
}
#text{
  position: absolute;
  top: 50%;
  left: 50%;
  font-size: 50px;
  color: white;
  transform: translate(-50%,-50%);
  -ms-transform: translate(-50%,-50%);
}
</style>
</head>
<body>
<div id="overlay" onclick="off()">
  <div id="text">Overlay Text</div>
</div>
<div style="padding:20px">
  <h2>Overlay with Text</h2>
  <button onclick="on()">Turn on overlay effect</button>
</div>
<script>
function on() {
  document.getElementById("overlay").style.display = "block";
}
function off() {