html - justify-content: space-between failing to align elements as expected

Question

I needed to use flexbox to center my navigation and hence I came up with the following:

.navbar-brand > img {
  width: 100px;
}
.navbar-default {
  background-color: #fff;
  border-color: #fff;
  -webkit-box-shadow: 0 0 3px rgba(0, 0, 0, .3);
  box-shadow: 0 0 3px rgba(0, 0, 0, .3);
}
.navbar-default .navbar-nav > li > a {
  color: #464646;
  text-transform: uppercase;
}
.navbar-default .navbar-nav > li > a:hover,
.navbar-default .navbar-nav > li > a:focus,
.navbar-default .navbar-nav > li > a:active {
  color: #727272;
}
.navbar-default .navbar-nav > li:not(.active) > a:before {
  content: '';
  position: absolute;
  bottom: 0;
  left: 30%;
  right: 30%;
  height: 1px;
  background: #ed1c24;
  opacity: 0;
  -webkit-transform: translateY(10px);
  -ms-transform: translateY(10px);
  -o-transform: translateY(10px);
  transform: translateY(10px);
  -webkit-transition: all .3s;
  -o-transition: all .3s;
  transition: all .3s;
}
.navbar-default .navbar-nav > li > a:hover:before {
  opacity: 1;
  -webkit-transform: none;
  -ms-transform: none;
  -o-transform: none;
  transform: none;
}
.navbar-default .navbar-nav > li:first-child > a {
  font-weight: 700;
}
.navbar-default .navbar-nav > li.active > a {
  background: #ed1c24;
  color: #fff;
  padding-top: 25px;
  padding-bottom: 25px;
  position: relative;
  -webkit-transition: all .3s;
  -o-transition: all .3s;
  transition: all .3s;
}
.navbar-default .navbar-nav > li.active > a:hover,
.navbar-default .navbar-nav > li.active > a:focus,
.navbar-default .navbar-nav > li.active > a:active {
  background: #e0222a;
  color: #fff;
}
.navbar-default .navbar-nav > li.active:hover > a:after {
  bottom: 0;
}
.navbar-default .navbar-nav > li.active > a:after {
  font-family: FontAwesome;
  content: 'f078';
  position: absolute;
  bottom: 5px;
  font-size: 0.6em;
  /*opacity: 0.8;*/
  left: 50%;
  -webkit-transform: translateX(-50%);
  -ms-transform: translateX(-50%);
  -o-transform: translateX(-50%);
  transform: translateX(-50%);
  -webkit-transition: all .3s;
  -o-transition: all .3s;
  transition: all .3s;
}
/* use flexbox to center align nav elements above 992px */

@media (max-width: 992px) {
  .navbar-default .navbar-nav > li > a {
    text-align: center;
  }
  .navbar-collapse {
    max-height: 350px;
    overflow-y: hidden;
  }
}
@media (min-width: 992px) {
  .navbar-default {
    display: flex;
    align-items: center;
    justify-content: space-between;
  }
  .navbar-default {
    min-height: 100px;
  }
  .navbar-default .navbar-right {
    display: flex;
    align-items: center;
  }
  .navbar-default > .container {
    display: flex;
    align-items: center;
    justify-content: space-between;
  }
}

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" rel="stylesheet" />
<nav role="navigation" class="navbar navbar-default navbar-fixed-top">
  <div class="container">
    <!-- Brand and toggle get grouped for better mobile display -->
    <div class="navbar-header">
      <button type="button" data-target="#navbarCollapse" data-toggle="collapse" class="navbar-toggle">
        <span class="sr-only">Toggle navigation</span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
      </button>
      <a href="index.html" class="navbar-brand">
        <img src="https://upload.wikimedia.org/wikipedia/commons/4/48/EBay_logo.png" alt="Logo">
      </a>
    </div>
    <!-- Collection of nav links and other content for toggling -->
    <div id="navbarCollapse" class="collapse navbar-collapse">
      <ul class="nav navbar-nav navbar-right">
        <li class="active"><a href="index.html">Home</a>
        </li>
        <li><a href="consulting.html">CONSULTING</a>
        </li>
        <li><a href="devices.html">Medical Devices</a>
        </li>
        <li><a href="">Servises</a>
        </li>
        <li><a href="">News</a>
        </li>
        <li><a href="">Contact Us</a>
        </li>
      </ul>
    </div>
  </div>
</nav>

Answer 1

The problem is a conflict with the Bootstrap stylesheet, which places pseudo-elements in your flex container. This causes space-between to calculate multiple flex items as opposed to just two.

Here's your flex container:

The logo and nav menu are aligned with justify-content: space-between, but are not positioned at opposite edges. The alignment looks more like space-around.

---

Here's Bootstrap's ::before and ::after pseudo-elements (or pseudo-flex items):

As noted in Firefox documentation:

In-flow ::after and ::before pseudo-elements are flex items.

---

Let's put some content in the pseudos:

Like shining a black light in a motel room, you see a lot of stuff you wish wasn't there.

---

Remove (or override) the pseudo-elements and your problem is gone:

---

More details about flex containers and pseudo-elements:

• Pseudo elements breaking justify-content: space-between in flexbox layout
• Properly sizing and aligning the flex item(s) on the last row
• Methods for Aligning Flex Items along the Main Axis (see Box #81)