Try it on any J2EE compliant container, where XML parsing of web.xml is enabled. It will surely give error.
Though the element which come directly under <web-app> , You can place the elemnt in any order sythatically. But the order of the sub-element under these elements is still important in Servlet 2.4 specifications. Tomcat will not give the error at it does not parse the web.xml at the time of deployment.