amis/scss/components/form/_fieldset.scss

%fieldSetBase {
  position: relative;

  &:after {
    content: '';
    pointer-events: none;
    border: 1px solid $borderColor;
    position: absolute;
    z-index: 0;
    top: 11px;
    left: 0;
    bottom: 0;
    right: 0;
  }

  > legend {
    position: absolute;
    top: 0;
    left: 0;
    display: inline-block;
    border: 0;
    width: auto;
    z-index: 1;
    background: $Fieldset-legend-bgColor;
    border-left: 0 !important;
  }

  .collapse {
    position: relative;
  }
}

fieldset.#{$ns}Collapse {
  > legend {
    font-weight: $fontWeightNormal;
    padding: $gap-xs 0;
    font-size: $fontSizeMd;
    color: $text--loud-color;
    border-left: $primary px2rem(4px) solid;
    line-height: 1.2;
    margin: 8px 0 28px;
    padding: 0 0 0 14px;
    cursor: pointer;
    border-bottom: 0;
  }

  &--xs {
    @extend %fieldSetBase;
    padding: 20px 5px 5px 5px;

    > legend {
      left: 5px;
      font-size: $fontSizeXs;
      padding: 0 3px;
      margin: 0 0 0 -3px;
    }

    &:after {
      top: 6px;
    }
  }

  &--sm {
    @extend %fieldSetBase;

    padding: 25px 10px 10px 10px;

    > legend {
      left: 10px;
      font-size: $fontSizeSm;
      padding: 0 5px;
      margin: 0 0 0 -5px;
    }

    &:after {
      top: 6px;
    }
  }

  &--base {
    @extend %fieldSetBase;

    padding: 30px 15px 15px 15px;

    > legend {
      left: 15px;
      font-size: $fontSizeBase;
      padding: 0 8px;
      margin: 0 0 0 -8px;
    }

    &:after {
      top: 7px;
    }
  }

  &--md {
    @extend %fieldSetBase;
    padding: 30px 20px 20px 20px;

    > legend {
      left: 20px;
      font-size: $fontSizeMd;
      padding: 0 10px;
      margin: 0 0 0 -10px;
    }

    &:after {
      top: 7px;
    }
  }

  &--lg {
    @extend %fieldSetBase;
    padding: 40px 30px 30px 30px;

    > legend {
      left: 30px;
      font-size: $fontSizeLg;
      padding: 0 15px;
      margin: 0 0 0 -15px;
    }

    &:after {
      top: 9px;
    }
  }
}