Simple retina mixin from 37signals

gistfile1.sass

// Mixin (1.3 due to Nexus 7)
@mixin image-2x($image, $width, $height) {
	@media (min--moz-device-pixel-ratio: 1.3),
	(-o-min-device-pixel-ratio: 2.6/2),
	(-webkit-min-device-pixel-ratio: 1.3),
	(min-device-pixel-ratio: 1.3),
	(min-resolution: 1.3dppx) {
		background-image: url($image);
		background-size: $width $height;
	}
}

// In use
input[type="email"] {
	background: url(mail.png) 14px 14px no-repeat;
	background-size: 21px 16px;
	@include image-2x("mail@2x.png", 21px, 16px);
}