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http://url.spec.whatwg.org/#concept-urlencoded-serializer Item 3.1 says: "Replace pair's name and value with the result of running encoding override's encoder on them, with encoding override's encoder's error handling mode set to <form>, respectively." Text "running" here must be link to Run process in Encoding spec (http://encoding.spec.whatwg.org/#concept-encoding-run). Otherwise it is interpeted same as "invoking", what sounds wrong because encoding's handler doesn't take 'error mode' parameter.
https://github.com/whatwg/url/commit/f7ab990492ff6f6f69b557b7693149f42bba6bd8