As far as I know, it's a bunch of specifications for Java, of which there are probably a few reference implementations and for the popular specs several competing implementations (Tomcat, Jetty, Glassfish/Payara, Hibernate, EclipseLink, ...)
The specification part means that in theory one can take code written for one container runtime and move it to another that offers better $nonFunctionalRequirement (or licensing story) without code changes
It's Jakarta Faces now ;) And it's still relative active with a major new version (Jakarta Faces 4) in Jakarta EE 10. Of course the entire concept of server side rendering is not as much used as it once was, but that said it's still used.
The specification part means that in theory one can take code written for one container runtime and move it to another that offers better $nonFunctionalRequirement (or licensing story) without code changes