[gsnedders]

To be clear, ADE’s behaviour is not conforming to any version of the standards it claims to implement. If it had been, it would reject that specific max-width property declaration as having an invalid value and ignore it, not reject the entire document: every single version of CSS has required that forwards-compatible behaviour.

PDF is not somehow immune to this either — a non-conforming implementation could similarly break what are meant to be forward-compatible extension points by raising an error on an unknown stream or object instead of (as required by the standard) ignoring it.

[anenefan]

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.

[gsnedders]

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!