web.xml 使用 Servlet4.0 版本，No grammar constraints (DTD or XML Schema) referenced in the document.
添加 <!DOCTYPE html>
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html>
<web-app version="4.0"
xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd">
</web-app>No grammar constraints (DTD or XML Schema) referenced in the document.
原文：https://www.cnblogs.com/wumz/p/10720976.html