Prova tu stesso - W3.CSS modal

<!DOCTYPE html>
<html>
<title>W3.CSS</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="/lib/w3.css">
<body class="w3-container">

<h2>W3.CSS Modal</h2>
<p>In this example we demonstrate how to close the modal by clicking outside of the modal box.</p>

<button onclick="document.getElementById('id01').style.display='block'" class="w3-btn">Open Modal</button>

<div id="id01" class="w3-modal">
  <div class="w3-modal-content w3-card-8">
    <header class="w3-container w3-teal"> 
      <span onclick="document.getElementById('id01').style.display='none'" 
      class="w3-closebtn">&times;</span>
      <h2>Modal Header</h2>
    </header>
    <div class="w3-container">
      <p>You have two options to close this modal:</p>
      <p>Click on the "x" or click anywhere outside of the modal!</p>
    </div>
    <footer class="w3-container w3-teal">
      <p>Modal Footer</p>
    </footer>
  </div>
</div>

// When the user clicks anywhere outside of the modal, close it
window.onclick = function(event) {
    if (event.target == modal) {
        modal.style.display = "none";
    }
}
</script>

</body>
</html>