Note that EPUB 2.1 specifies only a subset of CSS2 that's "required" by compliant rendering apps. So you may say that an app/software that doesn't support some of it is "bad", but I wouldn't dare saying anything about one that doesn't support stuff that's out of the specification. Sure, it's nice to have it, but one could blame the book as well as the app if it doesn't work (especially if the book relies on that, instead of using it as an optional enhancement).
(Disclaimer: Not talking about EPUB 3 here.)
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