With jetty12 you need to perform some extra configuration step.

  java -jar $JETTY_HOME/start.jar --add-modules=ee8-deploy
  systemctl restart jetty12

Notice: at the time of writing this, jetty12 is not completely functional in
Debian yet. See:
- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1108253
- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1108370


Once jetty12 is properly configured, css-validator webapp is reachable at:

  http://localhost:8080/css-validator