.border-box { box-sizing: border-box; }
.content-box { box-sizing: content-box; }
.box-initial { box-sizing: initial; }
.box-inherit { box-sizing: inherit; }

@each $breakpoint in $breakpoints {
  $bp: nth($breakpoint, 1);
  $letters: nth($breakpoint, 2);

  @media #{$bp} {
    .border-box-#{$letters} { box-sizing: border-box; }
    .content-box-#{$letters} { box-sizing: content-box; }
    .box-initial-#{$letters} { box-sizing: initial; }
    .box-inherit-#{$letters} { box-sizing: inherit; }
  }
}