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In http://dev.w3.org/csswg/css-syntax/#consume-a-token , when the input code point consumed is a digit, the spec indicates: "Consume a numeric token, and return it." The numeric token consumption starts from the next unconsumed input code point, resulting in the first digit being discarded and not added to the numeric token. Other code points that lead to "consume a numeric token" (e.g. '+', '-', '.') indicate to reconsume the current code point before calling "consume a numeric token". I think that the "digit" code point should include similar instructions.
Thanks for reporting! This was a spec bug, fixed in https://dvcs.w3.org/hg/csswg/rev/8663b0d5bbf0 This is a nice example of why we want *independent* implementations to validate the spec. I implemented this twice without noticing the bug, because I knew what the spec was supposed to say.