In a DTD, Attributes are declared with an ATTLIST declaration.在DTD中，属性是通过ATTLIST声明来声明的。
An attribute declaration has the following syntax:属性声明的语法如下：
<!ATTLIST element-name attribute-name attribute-type default-value> example: DTD example:<!ATTLIST payment type CDATA "check">XML example:<payment type="check" />The attribute-type can have the following values:属性类型含有以下值：
Value值Explanation解释CDATA
The value is character data字符数据值
(en1|en2|..)
The value must be one from an enumerated list必须是来自列表中的一个值
ID
The value is a unique id 唯一独立的id值
IDREF
The value is the id of another element其它元素的id值
IDREFS
The value is a list of other ids其它id的列表值
NMTOKEN
The value is a valid XML name其值为一个有效的XML名称
NMTOKENS
The value is a list of valid XML names其值为一组有效的XML名称列表值
ENTITY
The value is an entity 其值为一个实体
ENTITIES
The value is a list of entities其值为一组实体的列表
NOTATION
The value is a name of a notation其值为一个符号的名称
xml:
The value is a predefined xml value预定的XML值
The default-value can have the following values:省略补充含有以下值：
Value值Explanation解释value
The default value of the attribute属性默认值
#REQUIRED
The attribute value must be included in the element包含于元素内的属性值
#IMPLIED
The attribute does not have to be included可包含也可不包含于元素内
#FIXED value
The attribute value is fixed固定的属性值
In the example above, the "square" element is defined to be an empty element with a "width" attribute of type CDATA. If no width is specified, it has a default value of 0.在上面的例子中，"square"元素定义为含有CDATA类型的"width"属性的空元素。如果没有指定width值，那它默认为0。
Use the #IMPLIED keyword if you don't want to force the author to include an attribute, and you don't have an option for a default value.如果你不想让author元素包含一个属性值或对默认值不具备选择权，那么你不妨使用#IMPLIED关键字。
Use the #REQUIRED keyword if you don't have an option for a default value, but still want to force the attribute to be present.如果你对默认值不具备选择权却想却有想让属性值存在，那么你不妨使用#REQUIRED关键字。
Use the #FIXED keyword when you want an attribute to have a fixed value without allowing the author to change it. If an author includes another value, the XML parser will return an error.当你希望属性是一个固定值且不希望让author元素去改变它，我们建议你使用#FIXED关键字。如果author包含另外一个值，XML剖析器将会提示错误。
Use enumerated attribute values when you want the attribute values to be one of a fixed set of legal values.当你希望得到一个固定合法的属性值时，请使用列举属性值（Enumerated attribute values）的方法。
