If you are using Java 9 or later the core also depends on the JAXB API. This used to be part of the standard class library but has been split out of it with Java 9.
If you want to use Input.fromJaxb - i.e. you want to serialize plain Java objects to XML as input - then you also need to add a dependency on the JAXB implementation. Starting with XMLUnit 2.6.4, xmlunit-core optionally depends on the JAXB reference implementation and its transitive dependencies. Starting with XMLUnit 2.8.0 the JAXB dependency requires the JakartaEE version of JAXB.