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http://lists.w3.org/Archives/Public/www-style/2009Dec/0249.html
The part I am unclear about is how the invalid @keyframes rule specified in this prose is created in the first place given that section 3 specifies how to create from/0% or to/100% rules if they are not specified.
The language quoted in the mail is no longer present; given that the UA is required to create the missing 0%/100% frames I do not believe this is an issue anymore.