With tomcat11 you need to perform some extra configuration step.

  1. install `tomcat-jakartaee-migration` package if not done yet
  2. and then one of these:
      - add ",/usr/share/tomcat-jakartaee-migration/lib/*.jar" to
        common.loader in '/etc/tomcat11/catalina.properties'
      - create a link to /usr/share/tomcat-jakartaee-migration/lib/jakartaee-migration-1.0.6-shaded.jar
        in /usr/share/tomcat11/lib

Notice: at the time of writing this, there is a bug report on this. See:
- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1108280

With tomcat11, css-validator webapp is reachable at:

  http://localhost:8080/css-validator