Codebase Browser
chromium
Go to App

chromium/third_party/blink/web_tests/external/wpt/custom-elements/resources/navigation-destination.html 

<!DOCTYPE html>
 <html>
 <body>
 <p>Navigated!</p>
 <script>
 parent.postMessage('didNavigate', '*');
 </script>
 </body>
 </html>