http://dev.w3.org/csswg/cssom/#the-cssstyledeclaration-interface

[[
If property is an ASCII case-insensitive match for a property of a declaration in the declarations, and that declaration's property is defined to be case-insensitive, return the serialization of that declaration's value.

If property is a case-sensitive match for a property of a declaration in the declarations, and that declaration's property is defined to be case-sensitive, return the serialization of that declaration's value.
]]

Variables have their own interface so CSSOM can assume that all properties are case-insensitive. Possibly normalize the case to lowercase.