Use of "bl" class

HTML

<script src="https://code.jquery.com/jquery-3.4.1.slim.js"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.0/umd/popper.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/js/bootstrap.js"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.css">
  <nav class="navbar navbar-expand-lg navbar-dark" style="background-color: #333;" >
    <a class="navbar-brand" href="/">
      <img src="/static/img/AdiAvi.png" width="30" height="30" class="d-inline-block align-top" alt="">
      AdiAvi
    </a>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
      <span class="navbar-toggler-icon"></span>
    </button>
    <div class="collapse navbar-collapse" id="navbarNav">
      <ul class="navbar-nav ms-auto mb-2 mb-lg-0">
        <li class="nav-item">
          <a class="nav-link"  href="/">Home</a>
        </li>
        <li class="nav-item">
          <a class="nav-link" href="/contact">Contact</a>
        </li>
        <button type="button" id="loginButton" class="ml-2 btn btn-danger navbar-btn">
          Login
      </button>
        <button type="button" id="registerTrigger" class="ml-2 btn btn-primary navbar-btn">
            Register
        </button>

      </ul>
    </div>
  </nav>