Use Font Awesome icon as CSS content

If you have access to SCSS files from font-awesome, you can use this simple solution:

.a:after {
    // Import mixin from font-awesome/scss/mixins.scss
    @include fa-icon();

    // Use icon variable from font-awesome/scss/variables.scss
    content: $fa-var-exclamation-triangle;
}

You should have font-weight set to 900 for Font Awesome 5 Free font-family to work.

This is the working one:

.css-selector::before {
   font-family: 'Font Awesome 5 Free';
   content: "\f101";
   font-weight: 900;
}

a:before {
     content: "\f055";
     font-family: FontAwesome;
     left:0;
     position:absolute;
     top:0;
}

Example Link: https://codepen.io/bungeedesign/pen/XqeLQg

Get Icon code from: https://fontawesome.com/cheatsheet?from=io

You can use unicode for this in CSS. If you are using font awesome 5, this is the syntax;

.login::before {
    font-family: "Font Awesome 5 Free"; 
    font-weight: 900; 
    content: "\f007";  
}

You can see their documentation here.

As it says at FontAwesome website FontAwesome =>

HTML:

<span class="icon login"></span> Login</li>

CSS:

 .icon::before {
    display: inline-block;
    font-style: normal;
    font-variant: normal;
    text-rendering: auto;
    -webkit-font-smoothing: antialiased;
  }

    .login::before {
        font-family: "Font Awesome 5 Free";
        font-weight: 900;
        content: "\f007";
   }

In .login::before -> edit content:''; to suit your unicode.

Update for Font Awesome 5 using SCSS

.icon {
  @extend %fa-icon;
  @extend .fas;

  &:before {
    content: fa-content($fa-var-user);
  }
}

Here's my webpack 4 + font awesome 5 solution:

webpack plugin:

new CopyWebpackPlugin([
    { from: 'node_modules/font-awesome/fonts', to: 'font-awesome' }
  ]),

global css style:

@font-face {
    font-family: 'FontAwesome';
    src: url('/font-awesome/fontawesome-webfont.eot');
    src: url('/font-awesome/fontawesome-webfont.eot?#iefix') format('embedded-opentype'),
    url('/font-awesome/fontawesome-webfont.woff2') format('woff2'),
    url('/font-awesome/fontawesome-webfont.woff') format('woff'),
    url('/font-awesome/fontawesome-webfont.ttf') format('truetype'),
    url('/font-awesome/fontawesome-webfont.svgfontawesomeregular') format('svg');
    font-weight: normal;
    font-style: normal;
}

i {
    font-family: "FontAwesome";
}