DTD（文档类型定义）的作用是定义 XML 文档的合法构建模块。
DTD 可被成行地声明于 XML 文档中，也可作为一个外部引用。
| <!DOCTYPE NEWSPAPER [ |
| <!ELEMENT NEWSPAPER (ARTICLE+)> |
| <!ELEMENT ARTICLE (HEADLINE,BYLINE,LEAD,BODY,NOTES)> |
| <!ELEMENT HEADLINE (#PCDATA)> |
| <!ELEMENT BYLINE (#PCDATA)> |
| <!ELEMENT LEAD (#PCDATA)> |
| <!ELEMENT BODY (#PCDATA)> |
| <!ELEMENT NOTES (#PCDATA)> |
| <!ATTLIST ARTICLE AUTHOR CDATA #REQUIRED> |
| <!ATTLIST ARTICLE EDITOR CDATA #IMPLIED> |
| <!ATTLIST ARTICLE DATE CDATA #IMPLIED> |
| <!ATTLIST ARTICLE EDITION CDATA #IMPLIED> |
| ]> |
