DTD definitions
A DTD allows you to create new tags by writing grammar rules which the tags must obey. The rules specify which tags and attributes are valid and their context.
A DTD element declaration looks like: <!ELEMENT person(name, email*)>
- ELEMENT is the type
- person is the element declaration
- (name,email*) is the element content model
- name and email are the children of person and define the hierarchy of the document.
- Note that this is called a grammar rule because it could have been written in BNF: person ::= (name, email*)