I don't think we need this filter: property here. the opacity rule is pretty commonly understood by most agents and this filter one is less supported and redundant. If you were trying for IE support, this older -ms-filter solution is the correct syntax - http://msdn.microsoft.com/en-us/library/ie/ms530752%28v=vs.85%29.aspx. I wouldn't worry about implementing it, we have not added to other instances where opacity is defined.