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From Daniel: "On a similar note, the keyText attribute of CSSKeyframeRule is read/write. That means it is possible to programmatically assign a CSSKeyframeRule's key to an existing key in the parent CSSKeyframesRule... Section 6.2.2 of the spec does not say what happens in that case and I think it should. In particular, what happens: - if the assigned keyText is invalid ; should it throw anything? - if the assigned keyText already exists Same thing about CSSKeyframeRule.appendRule(). If the key specified by the passed string already exists in cssRules, I suppose the existing Keyframe should be deleted. This is not specified in the current ED."
Follow up at http://lists.w3.org/Archives/Public/www-style/2014Mar/0517.html
Throw syntax error per WG resolution from 2014-04-02 [1]: RESOLVED: Keytext on setting invalid value should throw an error. [1] http://lists.w3.org/Archives/Public/www-style/2014Apr/0016.html
Per WG resolution [1], appendRule always appends a valid rule at the end of the @keyframes rule. The rule's key is not considered. [1] http://log.csswg.org/irc.w3.org/css/2014-10-27/#e484268