It was removed from the specification in 2007.  Mozilla also removed support. 
https://bugs.webkit.org/show_bug.cgi?id=48289 was raised for similar issue. If we remove this feature, both of these would be resolved.
As per discussion on https://bugs.webkit.org/show_bug.cgi?id=48288, we would not remove "darker" support. We can mark the bug as WONTFIX
Closing per discussion on bug 48288.