6.1 Well Formed and Valid Documents
 6.2 Element Declarations
 6.3 Attribute Declarations
 6.4 Parsed and Unparsed Data
 6.5 External Resource Identifiers and Notation Declarations
 6.6 Entity Declarations
 6.7 Location of Entity Declarations
 6.8 Parsed and Unparsed Entities
 6.9 Comments
 6.10 Conditional Sections
 6.11 A DTD Example: XHTML Formalized
 6.12 BNF and EBNF Notations
 6.13 An EBNF Example: XML and DTD Formalized