DTD的ATTLIST没有效果

定义了xml和dtd文件，但是dtd的属性约束没有效果。

xml 文件

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE books SYSTEM "books2.dtd">
<books>
    <book id="a123" city="阿斯顿">
        <name>
        </name>
        <author>
        </author>
        <price>
        123
        </price>
    </book>
    <book id="a123">
        <name>
        </name>
        <author>
        </author>
        <price>
        321
        </price>
    </book>
</books>

下面是dtd文件

<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT books (book+)>
<!ELEMENT book (name?,author,(price|money))>
<!ELEMENT name (#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT price (#PCDATA)>
<!ATTLIST book
    id ID #REQUIRED
    title CDATA #IMPLIED
    lang CDATA #FIXED　"固定值"
    category  CDATA "默认值"
    pId IDREF #REQUIRED
    city (北京|上海|广州) #REQUIRED
>