An element consists of open and close tags containing the element name surrounding the element content ” <ElementName>element content</ElementName>.
Attributes appear in the open tag and consist of an attribute name and an attribute value ”<ElementName attributeName = "attributeValue">.
For elements with element content, a DTD specifies the allowable child elements ”<!ELEMENT ElementName (Child1*, Child2?, Child3+)>.
*, ?, and + indicate the number of times an element may occur ”0 or more, 0 or 1, and 1 or more, respectively. The default is exactly 1.
For elements with data content, a DTD specifies the type of data it may contain ”<!ELEMENT ElementName (#PCDATA)>.
For elements with no content, a DTD specifies an empty content model ”<!ELEMENT ElementName EMPTY>.
For elements with attributes, a DTD specifies the list of allowable attributes ”
<!ATTLIST ElementName attributeName1 CDATA "default"> attributeName2 CDATA #REQUIRED attributeName3 (option1 option2) "option1">
Default attribute values appear within quotation marks.
DTDs can require documents to assign an attribute value explicitly with the "#REQUIRED" keyword.
DTDs can specify an enumerated list of allowable attribute values bounded by parentheses with options separated by vertical bars.
