It looks like the resource loading of e.g. the StyleEngine is not loading information simply using URL.openStream() or things like that but somehow differently. My use case is to run JavaFX inside an OSGi-Container which installs its own protocols URL#setURLStreamHandlerFactory() to e.g. support urls like this ("bundleresource://88.fwk605645/test.css").

While new URL("bundleresource://88.fwk605645/test.css") will throw an exception standard Java-Application it is perfectly valid in OSGi-Apps. I don't know how high the priority is for you to run on OSGi so the change to use URL.openStream to load stuff is probably not only a problem of the CSS-Engine