Documents can be validated against a given DTD to see if they conform to the desired format. The DTD defines the valid markup elements for a document type and what they mean. In other words, a DTD is a descriptive envelope for the contents of the document. This DTD defines a document type article as consisting of two structural parts, author and text. Finally, the text element has an attribute, style.
Attributes modify the elements by carrying additional information about the document section.
The tags in this example are in bold and the attribute is in italic. There is no physical manifestation presumed by the style attribute value. These attributes describe how elements should be transformed by an application attempting to create mapped output. That is, the DTD designer associates fixed SDA attributes and values with each element to be transformed. After the markup and validation process is complete, the SGML document is handed over to the document output system, computer software that creates a publisher-determined output form from the SGML document.