Since you are using the v4.*.* of font-awesome the prefix class of invoking your desired icon is fa not fab . fab and fas are for the brand ... ... <看更多>

"fas fa-envelope", "far fa-envelope",. "fas fa-envelope-open", "far fa-envelope-open",. "fas fa-envelope-square",. "fab fa-envira",. "fas fa-eraser",. ... <看更多>

<i class="fas fa-envelope"></i> <span class="fa-layers-counter" style="background:Tomato">1,419</span> </span> ... ... <看更多>