问题
Why fontawesome icon class is different
<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>回答1:
The fa prefix has been deprecated in version 5. The new default is the fas solid style and the fab style for brands.
Please Refer to this
https://fontawesome.com/how-to-use/on-the-web/referencing-icons/basic-use
回答2:
Chek out Font Awesome website LINK. This image has all the informations about prefixes so there is nothing more to add :)
Font Awesome 5.10.0 has duotone icons with new prefix fad.
来源：https://stackoverflow.com/questions/49913423/why-font-awesome-icon-class-is-different