Document Type Definitions

Document Type Definitions (DTDs) and XML Schemas specify the names of the elements and attributes and their use in documents.
The DOCTYPE definition in a document specifies the one to be used, e.g.<!DOCTYPE greeting SYSTEM "hello.dtd">

This sample DTD says that the element questionlist must have an attribute named test and contain only question elements.

<!ENTITY % Text           "CDATA">

<!ELEMENT questionlist (question)+>
<!ATTLIST questionlist
  test      %Text;
>
<!ELEMENT question (qtext|choice|comment|%Text)+>
<!ELEMENT qtext (%Text)>
<!ELEMENT choice (%Text)>
<!ATTLIST choice
  answer     %Text;  #IMPLIED
>
<!ELEMENT comment (%body.content)>