ลองตัวเอง

<!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>

<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">
   <div class="w3-container">
   <span onclick="document.getElementById('id01').style.display='none'" class="w3-closebtn">×</span>
   <p>Some text. Some text. Some text.</p>
   <p>Some text. Some text. Some text.</p>
   </div>
  </div>
</div>

</body>
</html>