So if I understand correctly a struggling epub viewer or ADE should skip css that it considers malformed - which means the reason my viewers have considered a epub to be not able to be viewed / corrupt / whatever it is for some other reason than more recent / current css implementation.

PDFs certainly can suck, more often those that will only work with abode's software and other viewers I've tried can not.

While plausible, I would suspect it’s more likely you’ve just run into bugs than forwards-compatible error behaviour — most ePubs don’t get anywhere near actually interesting CSS!