A simple mixin to wrap some CSS into a pixel-ration media query

_pixelratio.scss

@mixin pixel-ratio($ratio){
  @media  (-webkit-min-device-pixel-ratio: $ratio), /* Webkit browsers */
          (min--moz-device-pixel-ratio: $ratio),    /* old Firefox (prior to FF 16) */
          (min-resolution: #{$ratio}dppx),          /* the standard using dppx */
          (min-resolution: #{$ratio * 96}dpi){      /* fallback using dpi */
    @content;
  }
}