为什么fontawesome图标类是不同的
<script defer src="https://use.fontawesome.com/releases/v5.0.10/js/all.js" integrity="sha384-slN8GvtUJGnv6ca26v8EzVaR9DC58QEwsIk9q1QXdCU8Yu8ck/tL/5szYlBbqmS+" crossorigin="anonymous"></script>
<link href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.css" rel="stylesheet" />
<i class="fa fa-edit"></i>
<i class="fas fa-pencil"></i>
<i class="fab fa-facebook"></i>