sass mixin for hidpi responsive behavior

hidpi.scss

@mixin hidpi {
	@media only screen and (-webkit-min-device-pixel-ratio: 2),
		only screen and (   min--moz-device-pixel-ratio: 2),
		only screen and (     -o-min-device-pixel-ratio: 2/1),
		only screen and (        min-device-pixel-ratio: 2),
		only screen and (                min-resolution: 192dpi),
		only screen and (                min-resolution: 2dppx) { @content; }
}