dtd 属性#FIXED 属性值固定

<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE shopping [
	<!ELEMENT book (name,price,count,comment)>
	<!ELEMENT name (#PCDATA)>
	<!ELEMENT price (#PCDATA)>
	<!ELEMENT count (#PCDATA)>
	<!ELEMENT comment (#PCDATA)>
	
	<!--	
		comment标签的
		flag属性 字符串类型 固定值
	-->
	<!ATTLIST comment 
		flag CDATA #FIXED "0" 
	>
	
]>

<shopping>

	<book>
		<name>周易·参同契</name>
		<price>100</price>
		<count>3</count>
		<comment flag="0"></comment>					
	</book> 

	<book>
		<name>本草纲目</name>
		<price>200</price>
		<count>5</count>
		<!-- comment标签中有属性flag，那么flag的值就必须是"0" -->
		<comment flag="1"></comment>
	</book>

	<book>
		<name>普贤行愿品</name>
		<price>300</price>
		<count>7</count>
		<comment></comment><!-- comment标签中可以没有属性 -->		
	</book>
	
</shopping>