private static void validateRendererElement(Element element) {
        String idAttr = element.getAttribute("id");

        if (idAttr.equals(xmlRendererId) || idAttr.equals(jsonRendererId)) {
            throw new IllegalArgumentException(String.format("Renderer id %s is reserved for internal use", idAttr));
        }
    }
Disallow renderers named "XmlRenderer" or "JsonRenderer"
config-model/src/main/java/com/yahoo/vespa/model/container/xml/ContainerModelBuilder.java