The java.lang.module package specification [1] says, in multiple places, that both the set of observable modules and the default set of root modules are implementation-specific.

This was not the original intent. The definition was always meant to be part of the specification, so that conforming Implementations of the Java Platform would behave consistently. (This is despite a later CSR, in JDK 11, [2] which affirmed the implementation-specific view.)

We should revise the java.lang.module package specification to include complete and standard definitions of the set of observable modules and the default set of root modules.

[1] https://docs.oracle.com/en/java/javase/22/docs/api/java.base/java/lang/module/package-summary.html
[2] https://bugs.openjdk.org/browse/JDK-8205169