image-width/height 写在background 前面无法获取正确的图片尺寸

[yisibl]

.foo {
  width: image-width;
  height: image-height;
  background: -webkit-image-set(
                    url(../img/line.png) 1x,
                    url(../img/[email protected]) 2x) 50% 50% no-repeat;
}