When the web browser starts to work offline, the onoffline attribute triggers. You can try to run the following code to implement onoffline attribute −
Example
<!DOCTYPE html> <html> <body ononline = "onlineFunc()" onoffline = "offlineFunc()"> <script> function onlineFunc() { alert ("Working online!"); } function offlineFunc() { alert ("Workinf offline!"); } </script> <p>Got o the web browser menu bar and click the File menu. Select "Work Offline" and toggle between online and offline.</p> </body> </html>