image-rendering: pixelated;
But it's not supported yet:
https://developer.mozilla.org/en-US/docs/CSS/image-rendering#Browser_compatibility
these currently work, but only for downscaling:
image-rendering: -moz-crisp-edges; /* Firefox */
image-rendering: -o-crisp-edges; /* Opera */
image-rendering: -webkit-optimize-contrast;/* Webkit (non-standard naming) */
image-rendering: crisp-edges;
-ms-interpolation-mode: nearest-neighbor; /* IE (non-standard property) */