We'd still have to compile HV into the Java 6 byte code format. When compiling an application using HV on Java 8, one could take advantage of the constraints being marked as repeatable. When compiling an application using HV on Java 6, the @Repeatable meta-annotation wouldn't be present and only the current style could be used (annotation types not being present at runtime don't cause an error when loading the annotated class).

The nice thing is, that it isn't any difference from the HV engine perspective, whether a constraint is specified several times using the Java 8 style or using the current style, as the Java 8 compiler will represent repeated annotations via an instance of the container annotation in the bytecode just as if it had been given that way in the source code.