web.xml配置bug之提示The content of element type "web-app" must match "(icon?,display- name?,description?,distributable?,

错误：配置web.xml时，出现红色叉叉，提示

The content of element type "web-app" must match "(icon?,display-
  name?,description?,distributable?,context-param*,filter*,filter-mapping*,listener*,servlet*,servlet-
  mapping*,session-config?,mime-mapping*,welcome-file-list?,error-page*,taglib*,resource-env-
  ref*,resource-ref*,security-constraint*,login-config?,security-role*,env-entry*,ejb-ref*,ejb-local-ref*)".
 - No grammar constraints (DTD or XML schema) detected for the document.

原因大概就是：web.xml中标签顺序不符合web-app_2_3.dtd规范

web.xml头部配置：

<?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>

原因：文档内容标签顺序不符合web-app_2_3.dtd规范

解决方案

1：严格按照提示上的顺序排列

2：删除<!DOCTYPE 。。。。这段内容，即<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd">删除这行