discourse/app/assets/stylesheets/common/base/bbcode.scss

// Support for BBCode styles like colors and font sizes

span {
  &.bbcode-b {
    font-weight: bold;
  }
  &.bbcode-i {
    font-style: italic;
  }
  &.bbcode-u {
    text-decoration: underline;
  }
  &.bbcode-s {
    text-decoration: line-through;
  }
  // Font sizes
  @for $i from 4 through 40 {
    &.bbcode-size-#{$i} {
      font-size: #{$i}px;
    }
  }
}