Seems like a #13014-style approach would be better.

@cvrebert you can have both we defining the following mixin:

    @retina-displays: ~"only screen and (-webkit-min-device-pixel-ratio: 2), only screen and (min--moz-device-pixel-ratio: 2), only screen and (-o-min-device-pixel-ratio: 2/1),  only screen and (min-device-pixel-ratio: 2),  only screen and (min-resolution: 192dpi),only screen and (min-resolution: 2dppx)";

// retina display
.screen-retina(@rules) {

  @media @retina-displays
     { @rules(); }
}

@bassjobsen This currently fails to compile; see the Travis build output.

@cvrebert yes i see, i'm sorry for that. I see i commit three files (keyframes.less, progress-bars.less and mixins.less) by accidentally. And now i have to say sorry again because of i don't know howto fix this?

Passing on this for the time being, thanks though.