@mixin for-res($size) {
  @if $size == ldpi-up {
    // 2017 LG 34" Ultrawide
    @media
    (min-resolution: 90dpi),
    (min-resolution: 1dppx),
    (-webkit-min-device-pixel-ratio: 1),
    (min--moz-device-pixel-ratio: 1) {
      @content;
    }
  } @else if $size == mdpi-up {
    // Fire HD 8 Tablet
    @media
    (min-resolution: 120dpi),
    (min-resolution: 1.3dppx),
    (-webkit-min-device-pixel-ratio: 1.3),
    (min--moz-device-pixel-ratio: 1.3) {
      @content;
    }
  } @else if $size == hdpi-up {
    @media
    (min-resolution: 160dpi),
    (min-resolution: 2dppx),
    (-webkit-min-device-pixel-ratio: 1.5),
    (min--moz-device-pixel-ratio: 1.5) {
      @content;
    }
  } @else if $size == xhdpi-up {
    // iPhone 7
    @media
    (min-resolution: 240dpi),
    (min-resolution: 2dppx),
    (-webkit-min-device-pixel-ratio: 2),
    (min--moz-device-pixel-ratio: 2) {
      @content;
    }
  } @else if $size == xxhdpi-up {
    // iPhone X (listed as having a pixel density of 462.63 but
    // -webkit-min-device-pixel-ratio is 3) ¯\_(ツ)_/¯
    @media
    (min-resolution: 320dpi),
    (min-resolution: 3dppx),
    (-webkit-min-device-pixel-ratio: 3),
    (min--moz-device-pixel-ratio: 3) {
      @content;
    }
  } @else if $size == xxxhdpi-up {
    @media
    (min-resolution: 480dpi),
    (min-resolution: 4dppx),
    (-webkit-min-device-pixel-ratio: 4),
    (min--moz-device-pixel-ratio: 4) {
      @content;
    }
  }
}

// usage

label {
  font-weight: 300;
  @include for-res(xhdpi-up) {
    font-weight: 100;
  }
}