2.3和2.4：
<?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>
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>
web.xml的schema文件
原文地址：http://www.cnblogs.com/aigeileshei/p/7611164.html