<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<body>

<p>Click the button to display the doctype systemId of the document.</p>

<button onclick="myFunction()">Try it</button>

<p><strong>Note:</strong> In Internet Explorer 8 and earlier, the doctype property returns <em>null</em> for HTML and XHTML documents, and will only work for XML documents.</p>

<p id="demo"></p>

<script>
function myFunction() {
  var x = document.doctype.publicId;
  window.alert(x)