Mixin: .pxrt()
Params: pixel ration (number) or mixin
.pxrt( 1, { background-image:url('1.webp') }, 2, { background-image:url('2.webp') }, 3, { background-image:url('3.webp') } ); // 1x upload only 1.webp // retina 2x upload only 2.webp // retina 3x upload only 3.webp