I've seen questions that dealt with not having the "fa" class in addition to the icon class, I've seen 404s that weren't discovered until they checked console, I've seen IIS issues, etc. but none have been my issue. Is there anything that would specifically stop WAMP from allowing FA to render? I can see that the font file was loaded through dev console, I can see CSS being applied to the fa item and nothing's got an override. What I'm seeing if I hover over the element is a square shaped like the item highlighted, but nothing there. I changed the color to make sure it wasn't white-on-white but nothing shows. If I change the :before content to "Hello" I can see it written in a strange font. That's about all I can think to provide, any help would be appreciated. In my header:
<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet" integrity="sha384-wvfXpqpZZVQGK6TAh5PVlGOfQNHSoD2xbE+QkPxCAFlNEevoEH3Sl0sibVcOQVnN" crossorigin="anonymous">
And my icon:
<i class="fa fa-envelope-open-o" aria-hidden="true"></i>
Are you using https in WAMP? If not, then you are going to get security warnings since you are using https in the font awesome href. Try changing it to the following so it will be protocol independent and see if that works.
<link href="//maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet" integrity="sha384-wvfXpqpZZVQGK6TAh5PVlGOfQNHSoD2xbE+QkPxCAFlNEevoEH3Sl0sibVcOQVnN" crossorigin="anonymous">