The CSS specification clearly says that the CSS language itself is case insensitive. Unfortunately, it will be applied to data representing things such as the name of a class which is case sensitive. For this reason, it is important for csspp to keep input identifiers in the same case as it is found in the input file.
Version 1.0.7 includes that fix with tests used to verify that the code matches as expected.
Note that csspp still transforms function names to lowercase. This can cause problems with CSS used for Internet Explorer older versions (6 and earlier, versions that are not supported anymore and thus should not be of any concern).