老师我的约束不起作用 能帮我看下是为什么吗

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hr SYSTEM "hr.dtd">
<hr>
    <employee no="3309">
        <name>张三</name>
        <age>31</age>
        <salary>4000</salary>
        <department>
            <dname>会计部</dname>
            <address>XX大厦-8103</address>
        </department>
    </employee>
   
    <employee no="3310">
        <name>李四</name>
        <age>23</age>
        <salary>3000</salary>
        <department>
            <dname>工程部</dname>
            <address>XX大厦-8104</address>
        </department>
    </employee>
</hr>

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