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First of all, it's <url>, not <uri>. Second, and more important, it needs to include the string "url(" somewhere. And perhaps quotes, or some other form of escaping?
What the spec says... [[ <uri> The absolute URL URL escaped. ]] where "URL escaped" is http://dev.w3.org/csswg/cssom/#serialize-a-url which in turn uses http://dev.w3.org/csswg/cssom/#serialize-a-string So "url(" and quotes and ")" are included AFAICT. Keeping open for uri->url
Can we make the wording clearer, perhaps? Like: The absolute URL, serialized according to the "serialize a URL" rules. with "serialize a URL" a link?