From jboss-as-webservices12.xsd, all elements' minOccurs under webserivce subsystem is "0". When remove all the elements under subsytem, it fails to deploy webservice endpoint:
11:26:24,723 ERROR [org.jboss.msc.service.fail] (MSC service thread 1-1) MSC000001: Failed to start service jboss.deployment.unit."jaxws-samples-schemavalidation.war".INSTALL: org.jboss.msc.service.StartException in service jboss.deployment.unit."jaxws-samples-schemavalidation.war".INSTALL: WFLYSRV0153: Failed to process phase INSTALL of deployment "jaxws-samples-schemavalidation.war"
at org.jboss.as.server.deployment.DeploymentUnitPhaseService.start(DeploymentUnitPhaseService.java:163) [wildfly-server-9.0.0.Alpha1-SNAPSHOT.jar:9.0.0.Alpha1-SNAPSHOT]
at org.jboss.msc.service.ServiceControllerImpl$StartTask.startService(ServiceControllerImpl.java:1948) [jboss-msc-1.2.2.Final.jar:1.2.2.Final]
at org.jboss.msc.service.ServiceControllerImpl$StartTask.run(ServiceControllerImpl.java:1881) [jboss-msc-1.2.2.Final.jar:1.2.2.Final]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [rt.jar:1.7.0_17]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [rt.jar:1.7.0_17]
at java.lang.Thread.run(Thread.java:722) [rt.jar:1.7.0_17]
Caused by: java.lang.NullPointerException
at org.jboss.ws.common.deployment.EndpointMetricsDeploymentAspect.start(EndpointMetricsDeploymentAspect.java:49)
at org.jboss.as.webservices.deployers.AspectDeploymentProcessor.deploy(AspectDeploymentProcessor.java:73)
at org.jboss.as.server.deployment.DeploymentUnitPhaseService.start(DeploymentUnitPhaseService.java:156) [wildfly-server-9.0.0.Alpha1-SNAPSHOT.jar:9.0.0.Alpha1-SNAPSHOT]
... 5 more

11:26:24,736 ERROR [org.jboss.as.controller.management-operation] (Controller Boot Thread) WFLYCTL0014: Operation ("deploy") failed - address: ([("deployment" => "jaxws-samples-schemavalidation.war")]) - failure description: {"WFLYCTL0080: Failed services" =>

{"jboss.deployment.unit.\"jaxws-samples-schemavalidation.war\".INSTALL" => "org.jboss.msc.service.StartException in service jboss.deployment.unit.\"jaxws-samples-schemavalidation.war\".INSTALL: WFLYSRV0153: Failed to process phase INSTALL of deployment \"jaxws-samples-schemavalidation.war\" Caused by: java.lang.NullPointerException"}

}