<!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); } </script> </body> </html>