Notation:
<!ELEMENT name (content)>
Declaring Content:
Element has character data children (Parsed Character DATA)
<!ELEMENT name (#PCDATA)>
Empty Element (without child elements and text nodes)
<!ELEMENT name (EMPTY)>
Allow any child-element
<!ELEMENT name (ANY)>
Defines a child element
<!ELEMENT name (child-name)>
Defines an optional child element
<!ELEMENT name (child-name?)>
Defines one or more child elements
<!ELEMENT name (child-name+)>
Defines zero or more child elements
<!ELEMENT name (child-name*)>
Defines a sequence of child elements
<!ELEMENT name (child1, child2)>
Defines a choice of child elements
<!ELEMENT name (child1 | child2)>
Defines Mixed Content
<!ELEMENT name (child-name | #PCDATA)*>
Nesting Elements
<!ELEMENT name ((child1 | child2)+, child-name | #PCDATA)*>