operator
key of data-attribute of attribute-reference
    
The key of data-attribute operator returns the "true" name of the data-attribute. This is most useful when
        retrieving the real name of an data-attribute that is being referenced through an alias. The name is uppercased when
        the SGML declaration specifies NAMECASE GENERAL YES for the SGML document being
        processed.
      
Because data-attributes are always considered part of a shelf of attributes, it always makes sense to ask for
        the "key" of a data-attribute, even if the data-attribute form is used rather than
        data-attributes.