ul
  - for (var i = 1; i <= 13; i++)
    li(class='font-size-' + i) 
      | font-size: #{i}px

body {
  padding: 2vw;
  line-height: 1.8;
}

@for $i from 1 through 13 {
  .font-size-#{$i} {
    font-size: #{$i}px;
  }
}