Due to security problems, web browsers no longer support java applets. Some old versions of web browsers might work, e.g. Internet Explorer 11 unders Windows 10. See below for other suggestions to execute the java applets using Chrome.
Another option is to install the CheerpJ extension to Chrome. You can get more informaiton online and go to the Chromestore to download and install the extension. As of Spring 2021, CheerpJ is more reliable than the IE tab extension.