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CSS3 is good, but it's not exclusive of ePub3. Any ePub2 reader can support CSS3 and still be ePub2 compliant, any ePub2 book can use CSS3 and still be ePub2 compliant. I'd guess any reader that can support ePub2 and ePub3, supports the same amount of CSS3 in both formats.
That is not the case for some other ePub3 "features": include a <video> tag and your book stops being ePub2 compliant, drop support for NCX and your reader stops being ePub2 compliant.
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