dppx
Dots per CSS pixel — density spoken in the stylesheet's own tongue.
One dppx counts physical dots per CSS pixel: 1dppx is the classic screen, 2dppx the “retina” one, 3dppx the densest glass. It is the same number JavaScript hands over in devicePixelRatio — and the standard still registers x as its notarized alias.
It is the modern form of resolution queries: min-resolution: 2dppx says “dense screen” without making anyone multiply by 96 in their head. The right choice whenever the sheet must serve an image worthy of the glass.
@media (min-resolution: 2dppx) {
.mapa {
background-image: url(mapa@2x.png);
}
}