Servlet 2.3

<?xml version="1.0" encoding="UTF-8"?>  
<!DOCTYPE web-app  
    PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
    "http://java.sun.com/dtd/web-app_2_3.dtd">
<web-app>  
  ...
</web-app>  

Servlet 2.4

<?xml version="1.0" encoding="UTF-8"?>  
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
        xmlns="http://java.sun.com/xml/ns/j2ee"
        xmlns:web="http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
        xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
        version="2.4">
  ...
</web-app>  

Servlet 2.5

<?xml version="1.0" encoding="UTF-8"?>  
<web-app xmlns="http://java.sun.com/xml/ns/javaee"  
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
        version="2.5">
  ...
</web-app>  

JavaEE6 web.xml

<?xml version="1.0" encoding="UTF-8"?>  
<web-app xmlns="http://java.sun.com/xml/ns/javaee"  
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
          http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
         version="3.0">

</web-app>  

JavaEE7 web.xml

<?xml version="1.0" encoding="UTF-8"?>  
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:web="http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee">

</web-app>