// THIS FILE SHOULD OUTPUT NOTHING!
//
// As this file is included in many places (for variable/mixin access),
// it must output nothing for performance reasons.
//
// IF IT'S NOT A MIXIN OR VARIABLE, IT DOESN'T GO HERE!

// not the most up to date method, but works in IE7
.image-replaced() {
    text-indent: 120%; // extra 20% to account for fancy fonts that may overhang
    white-space: nowrap;
    overflow: hidden;
}

// A helper mixin for applying high-resolution background images (http://www.retinajs.com)
// File-names should follow:
//  example.png
//  example-high-res.png
@highdpi: ~"(-webkit-min-device-pixel-ratio: 1.5), (min--moz-device-pixel-ratio: 1.5), (-o-min-device-pixel-ratio: 3/2), (min-resolution: 1.5dppx)";
.at2x(@path, @w: auto, @h: auto) {
    background-image: url(@path);
    @at2x_path: ~`@{path}.replace(/\.\w+$/, function(match) { return "-high-res" + match; })`;
    @media @highdpi {
        background-image: url("@{at2x_path}");
        -webkit-background-size: @w @h;
                background-size: @w @h;
    }
}

// THIS FILE SHOULD OUTPUT NOTHING!
//
// As this file is included in many places (for variable/mixin access),
// it must output nothing for performance reasons.
//
// IF IT'S NOT A MIXIN OR VARIABLE, IT DOESN'T GO HERE!

// not the most up to date method, but works in IE7
.image-replaced() {
    text-indent: 120%; // extra 20% to account for fancy fonts that may overhang
    white-space: nowrap;
    overflow: hidden;
}

// A helper mixin for applying high-resolution background images (http://www.retinajs.com)
// File-names should follow:
//  example.png
//  example-high-res.png
@highdpi: ~"(-webkit-min-device-pixel-ratio: 1.5), (min--moz-device-pixel-ratio: 1.5), (-o-min-device-pixel-ratio: 3/2), (min-resolution: 1.5dppx)";
.at2x(@path, @w: auto, @h: auto) {
    background-image: url(@path);
    @at2x_path: ~`@{path}.replace(/\.\w+$/, function(match) { return "-high-res" + match; })`;
    @media @highdpi {
        background-image: url('@{at2x_path}');
        -webkit-background-size: @w @h;
                background-size: @w @h;
    }
}

Update @highdip to use single quotes
Update @highdip to use single quotes

As per CSS style guide.

Please note: I do not have LESS installed and cannot test this locally. Maybe it was done with double quotes to work around a LESS parsing limitation?