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SGML and XML Keywords
The previous SGML example declares the lang attribute as having values of type NAME, an alphabetic string. NAME is one of several SGML/XML keywords occurring in HTML and XHTML's declarations of an attribute's type:
CDATA Unparsed character data
ID A document-wide unique identifier
IDREF A reference to a document-wide identifier
NAME An alphabetic character string plus a hyphen and a period
NMTOKEN An alphanumeric character string plus a hyphen and a period
NUMBER A character string containing decimal numbers
Notice in early DTD fragment for <bdo> that the dir attribute does not declare its type using a keyword. Instead, the type is specified using an enumerated list containing two possible values, ltr and rtl. In that example for either SGML or XML, the attribute's default behavior is specified with a keyword. A default value may be specified using a quoted string.
#REQUIRED A value must be supplied for the attribute.
#IMPLIED The attribute is optional.
#FIXED The attribute has a fixed value that is declared in quotes using an additional parameter. Because the attribute/value pair is assumed to be constant, it does not need to be used in the document instance.
