Underline active navbar links in bootstrap 4 - css

Question

I'm upgrading a website from bootstrap 3 to 4 (beta). The css I used to underline active navbar links is no longer working. Instead of underlining the item, it underlines the full width of the page. How can I underline active nav items in bootstrap 4?
Bootstrap 3 css
.navbar-default .navbar-nav > .active:after {
position: absolute;
bottom: 0;
left: 0;
width: 100%;
content: " ";
border-bottom: 5px solid #5BC0EB;
}
Attempted Bootstrap 4 css
.navbar-light .navbar-nav .active::after {
position: absolute;
bottom: 0;
left: 0;
width: 100%;
content: " ";
border-bottom: 5px solid #5BC0EB;
}
HTML
<nav class="navbar fixed-top navbar-light bg-light navbar-expand-md">
<div class="container">
<a class="navbar-brand" href="#">My Website</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarCollapse" aria-controls="navbarCollapse"
aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse justify-content-end" id="main-nav-collapse">
<ul class="navbar-nav">
<li class="nav-item active"><a class="nav-link" href="#about">About</a></li>
<li class="nav-item"><a class="nav-link" href="#classes">Classes</a></li>
<li class="nav-item"><a class="nav-link" href="#gallery">Gallery</a></li>
<li class="nav-item"><a class="nav-link" href="#events">Events</a></li>
<li class="nav-item"><a class="nav-link" href="#contact">Contact</a></li>
<li class="nav-item"><a class="nav-link" href="#offers">Offers</a></li>
</ul>
</div>
</div>
</nav>

Answer 1

This happened because of your positioning.
Try to set relative position to your li and set after to a tag inside li which will be absolute positioned like below
.navbar-nav > li {
float: left;
position: relative;
}
.navbar-light .navbar-nav .active a::after {
border-bottom: 5px solid #5bc0eb;
bottom: -10px;
content: " ";
left: 0;
position: absolute;
right: 0;
}
Hope it helps.