没有起到约束作用，运行以后不报错。

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE test SYSTEM "test.dtd"> 
<!-- 人力资源管理系统 -->

<test>
    <employee number="3309">
        <name>张三</name>
        <age>31</age>
        <salary>4000</salary>
        <department>
            <dname>财务部</dname>
            <address>华瀚科技</address>
        </department>
    </employee>
    
    <employee number="3310">
        <name>李四</name>
        <age>23</age>
        <salary>3000</salary>
        <department>
            <dname>工程部</dname>
            <address>华瀚科技</address>
        </department>
    </employee>
    
</test>


<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT test (employee)>