Bug 888689 made us render custom mouse cursors using SVG images at the native HiDPI resolution. However, it's still not possible for raster image mouse cursors to be displayed at their native resolution on HiDPI. See bug 888689 for more information and a very helpful testcase.

How would this be specified on CSS?

CUR has support for multiple resolutions baked-in. See attachment 769575 [details] (from bug 888689) for a testcase. The CUR format always has a logical size of 32x32 and the best pixel size is meant to be subbed in. At least this is how Windows handles them and it's a proprietary Windows format anyway. PNG cursors will probably need to wait for image-set. Also please don't forget about bug 888781 (Windows), especially if it's convenient to fix them together.