Font-Awesome/web-fonts-with-css/scss/_icons.scss

/* Font Awesome uses the Unicode Private Use Area (PUA) to ensure screen
readers do not read off random characters that represent icons */

@each $fa-var-key, $fa-var-value in $fa-vars{
.#{$fa-css-prefix}-#{$fa-var-key}:before { content: fa-content($fa-var-value); }
}