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http://jigsaw.w3.org/css-validator/validator?text=a+%7B+color%3A+\20AC+% 7D&output=soap12 returns, on my system, the euro-sign Windows-1252 encoded, but does not declare that (or any other) encoding, http://jigsaw.w3.org/css-validator/validator?text=a+%7B+color%3A+\263A+% 7D&output=soap12 gives a question mark instead of a white smiling face.
Fixed, see http://qa-dev.w3.org:8001/css-validator/