Yes, there is the schema file. (web-app_2_4.xsd) which defines what elements can go into web.xml
http://java.sun.com/xml/ns/j2ee/ This is the equivalent of web-app_2_3.dtd that we had for Servlets2.3(J2EE1.3)
I guess the schema file is described in the Deployment Descriptor section in
Servlets Specs 2.4.(page:107)