Auto merge of #16553 - hiikezoe:vendor-prefix-keyframes, r=emilio

Support vendor prefix keyframes rule.

If there are multiple prefixed/non-prefixed @keyframes with the same name;

* non-prefixed rule overrides earlier rules.
* prefixed rule overrides earlier prefixed rules.

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This is PR of https://bugzilla.mozilla.org/show_bug.cgi?id=1356779

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