background: url(foo), red url(bar); gets serialized as "rgb(255, 0, 0) url(foo), url(bar) repeat, repeat scroll, scroll 0% 0%, 0% 0% / auto, auto padding-box, padding-box border-box, border-box" which doesn't match the spec: http://www.w3.org/TR/css3-background/#ltfinal-bg-layergt