Font Awesome 5, why don't brand icons work when using font-face?

Question

Imagine that I only have control through CSS. HTML is generated automatically, so I cannot add JS or change the HTML.

/*!
 * Font Awesome Free 5.13.0 by @fontawesome - https://fontawesome.com
 * License - https://fontawesome.com/license/free (Icons:CC BY 4.0, Fonts:SIL OFL 1.1, Code:MIT License)
 */

@font-face {
    font-family: 'my_font_awesome';
    font-style: normal;
    font-weight: 900;
    font-display: auto;
    src: url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.eot);
    src: url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.eot?#iefix) format('embedded-opentype'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.woff2) format('woff2'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.woff) format('woff'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.ttf) format('truetype'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.svg#fontawesome) format('svg');
}

#btn_apple:before,
#btn_check:before,
#btn_facebook:before,
#btn_plus:before {
  font-family: 'my_font_awesome';
  margin-right: .6rem;
  margin-left: 1rem;
}

#btn_apple:before {content:'\f179'}
#btn_check:before {content:'\f00c'}
#btn_facebook:before {content:'\f082'}
#btn_plus:before {content:'\f067'}

<a id="btn_apple">Apple</a>
<a id="btn_check">Check</a>
<a id="btn_facebook">Facebook</a>
<a id="btn_plus">Plus</a>

https://jsfiddle.net/johnlasantos/o1thrzxv/12/

Answer 1

You need to generate a new font for the brand icons because they don't belong to the same group as the solid one. Then you can simply use both font together and the browser will pick the needed one:

/*!
 * Font Awesome Free 5.13.0 by @fontawesome - https://fontawesome.com
 * License - https://fontawesome.com/license/free (Icons:CC BY 4.0, Fonts:SIL OFL 1.1, Code:MIT License)
 */

@font-face {
    font-family: 'my_font_awesome';
    font-style: normal;
    font-weight: 900;
    font-display: auto;
    src: url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.eot);
    src: url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.eot?#iefix) format('embedded-opentype'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.woff2) format('woff2'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.woff) format('woff'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.ttf) format('truetype'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-solid-900.svg#fontawesome) format('svg');
}

@font-face {
    font-family: 'my_font_awesome_b';
    font-style: normal;
    font-weight: 400;
    font-display: auto;
    src: url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-brands-400.eot);
    src: url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-brands-400.eot?#iefix) format('embedded-opentype'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-brands-400.woff2) format('woff2'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-brands-400.woff) format('woff'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/fa-brands-400.ttf) format('truetype'), url(https://use.fontawesome.com/releases/v5.13.0/webfonts/ffa-brands-400.svg#fontawesome) format('svg');
}

#btn_apple:before,
#btn_check:before,
#btn_facebook:before,
#btn_plus:before {
  font-family: 'my_font_awesome','my_font_awesome_b';
  margin-right: .6rem;
  margin-left: 1rem;
}

#btn_apple:before {content:'\f179'}
#btn_check:before {content:'\f00c'}
#btn_facebook:before {content:'\f082'}
#btn_plus:before {content:'\f067'}

<a id="btn_apple">Apple</a>
<a id="btn_check">Check</a>
<a id="btn_facebook">Facebook</a>
<a id="btn_plus">Plus</a>

More details from the Documentation: https://fontawesome.com/how-to-use/on-the-web/setup/hosting-font-awesome-yourself#using-certain-styles

Related question: Font Awesome 5 Choosing the correct font-family in pseudo-elements