Specialization rules for attributes

There are certain rules that apply to attribute specializations.

A specialized attribute has the following characteristics:

• It is specialized from @props or @base.
• It is declared as a global attribute. Attribute specializations cannot be limited to specific element types.
• It does not have values or value ranges that are more extensive than those of the attribute from which it was specialized.
• Its values must be alphanumeric space-delimited values. In generalized form, the values must conform to the rules for attribute generalization.