This legacy site uses Java Applets to run many of the calculations.
Web based Java functionality has been removed from all modern browsers. The applets can still be run using Java 8 Web Start or Iced Tea-Web.
Oracle still provides Java 8 but the license is limited to personal use only.
AdoptOpenJDK also provides an open source Java 8 SDK with an optional IcedTea-Web install for Windows.