fas icons definition code example
Example 1: font awesome icons
<link rel="stylesheet" href="path/to/font-awesome/css/font-awesome.min.css">
Example 2: fa icons
<script src='https://kit.fontawesome.com/a076d05399.js'></script>
<link rel="stylesheet" href="path/to/font-awesome/css/font-awesome.min.css">
<script src='https://kit.fontawesome.com/a076d05399.js'></script>