CSS FAQ
questions from implementers
That line defines what input is lexically incorrect. The statement that prints an error message is only a suggestion. What a parser should do when it reaches this rule depends on what application the parser is part of. In modern GUI-based aplications, printing an error on stderr is probably not the best solution. In a browser, the parser should probably just silently ignore the unexpected characters; there isn't much the reader can do about it anyway - that's the author's job.