Font Awesome is a collection of web icons that can be utilized via CSS or HTML.
Issues
Font Awesome 5 font-family not working
After upgrading to font awesome version 5 some icons may not work. An empty square may take their place. The cause of this problem is the lack of the font-weight CSS setting.
Solution[1]
Add font-weight: 900; to your CSS styling
Example
a::after {
content: "\f007"; font-family: 'Font Awesome\ 5 Free'; font-weight: 900;
}