Font-Awesome/js-packages/@fortawesome/fontawesome-free/scss/_mixins.scss

// mixins
// --------------------------

// base rendering for an icon
@mixin fa-icon {
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  display: inline-block;
  font-style: normal;
  font-variant: normal;
  font-weight: normal;
  line-height: 1;
}

// sets relative font-sizing and alignment (in _sizing)
@mixin fa-size ($font-size) {
  font-size: fa-divide($font-size, $fa-size-scale-base) * 1em; // converts step in sizing scale into an em-based value that's relative to the scale's base
  line-height: fa-divide(1, $font-size) * 1em; // sets the line-height of the icon back to that of it's parent
  vertical-align: (fa-divide(6, $font-size) - fa-divide(3, 8)) * 1em; // vertically centers the icon taking into account the surrounding text's descender
}

// only display content to screen readers
// see: https://www.a11yproject.com/posts/2013-01-11-how-to-hide-content/
// see: https://hugogiraudel.com/2016/10/13/css-hide-and-seek/
@mixin fa-sr-only() {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border-width: 0;
}

// use in conjunction with .sr-only to only display content when it's focused
@mixin fa-sr-only-focusable() {
  &:not(:focus) {
    @include fa-sr-only();
  }
}

// sets a specific icon family to use alongside style + icon mixins
@mixin fa-family-classic() {
  @extend .fa-classic;
}

// convenience mixins for declaring pseudo-elements by CSS variable,
// including all style-specific font properties
@mixin fa-icon-solid($fa-var) {
  @extend .fa-solid;

  & { #{$fa-icon-property}: unquote("\"#{ $fa-var }\""); #{$fa-duotone-icon-property}: unquote("\"#{ $fa-var }#{ $fa-var }\""); }
}
@mixin fa-icon-regular($fa-var) {
  @extend .fa-regular;

  & { #{$fa-icon-property}: unquote("\"#{ $fa-var }\""); #{$fa-duotone-icon-property}: unquote("\"#{ $fa-var }#{ $fa-var }\""); }
}
@mixin fa-icon-brands($fa-var) {
  @extend .fa-brands;

  & { #{$fa-icon-property}: unquote("\"#{ $fa-var }\""); #{$fa-duotone-icon-property}: unquote("\"#{ $fa-var }#{ $fa-var }\""); }
}