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http://lists.w3.org/Archives/Public/www-style/2011Nov/0608.html
Not sure how/whether to handle this; CSS3 Fonts does not say anything either. Where at-rules can be defined and their scope is not defined by these modules imo. Propose WONTFIX.
@keyframes rules are not handled any differently than @font-face, which does not specify its scope either. As for whether they can be nested inside @media or other at-rules, that is up to those modules to define whether and how it happens.