Auto merge of #16557 - hiikezoe:dont-support-moz-keyframes, r=SimonSapin

Servo does not need to support -moz-keyframes

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In #16553 I did innocently add a support for @-moz-keyframes but as @SimonSapin pointed out, servo does not need to support -moz prefix one.
Whereas, I think servo should support @-webkit-keyframes since it is described in web
compatibility [1].

[1] https://compat.spec.whatwg.org/#css-at-rules

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components/style/stylesheets.rs

@@ -1131,6 +1131,11 @@ impl<'a, 'b> AtRuleParser for NestedRuleParser<'a, 'b> {
                 } else {
                     None
                 };
+                if cfg!(feature = "servo") &&
+                   prefix.as_ref().map_or(false, |p| matches!(*p, VendorPrefix::Moz)) {
+                    // Servo should not support @-moz-keyframes.
+                    return Err(())
+                }
                 let name = match input.next() {
                     Ok(Token::Ident(ref value)) if value != "none" => Atom::from(&**value),
                     Ok(Token::QuotedString(value)) => Atom::from(&*value),