A must not occur. Either A or B must occur, but not both. Both A and B must occur, in any order. A few HTML element types use an additional SGML feature to exclude elements from their content model. Excluded elements are preceded by a hyphen.
Explicit exclusions override permitted elements.
The name of an attribute. The type of the attribute's value or an explicit set of possible values. Values defined explicitly by the DTD are case-insensitive. Please consult the section on basic HTML data types for more information about attribute value types. Some attribute definitions explicitly specify a default value for the attribute.