This attribute holds the DOI (Digital Object Identifier) assigned to the article. The reason why the attribute is not required, according to the DTD, is that the DOI might not be known when the article is initially tagged. But it is a required value by the time the article is published.

doi="10.1081/ADA-120029861"

Defined in shared attribute group: "Article Attributes"

This attribute is not required. It has text content.

The DTD contains the following attribute definition (note that embedded links are to definitions within the DTD page of this guide):

                doi                      CDATA  #IMPLIED