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

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

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

// sets relative font-sizing and alignment (in _sizing)
.fa-size(@font-size) {
  font-size: (@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: (1 / @font-size) * 1em; // sets the line-height of the icon back to that of it's parent
  vertical-align: ((6 / @font-size) - (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/
.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
.fa-sr-only-focusable() {
  &:not(:focus) {
    .fa-sr-only();
  }
}

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

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

  & { @{fa-icon-property}: @fa-var; @{fa-duotone-icon-property}: %("%s%s", @fa-var, @fa-var); }
}

.fa-icon-regular(@fa-var) {
  &:extend(.fa-regular all);

  & { @{fa-icon-property}: @fa-var; @{fa-duotone-icon-property}: %("%s%s", @fa-var, @fa-var); }
}

.fa-icon-brands(@fa-var) {
  &:extend(.fa-brands all);

  & { @{fa-icon-property}: @fa-var; @{fa-duotone-icon-property}: %("%s%s", @fa-var, @fa-var); }
}