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HTML

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    
    <meta name="description" content="">
    <meta name="author" content="">
    <link rel="icon" href="../../favicon.ico">

    <title>Vertically center element on HTML page</title>

    <!-- Bootstrap core CSS -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css">

<style>
body {
  padding-top: 50px;
}
.block {
  height: calc(100vh - 50px);
  background-color: darkorange;
}  
.block2 {
  height: 20vh;
  background-color: blueviolet;
} 
.centerthis {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  margin: auto;  
}
</style>

</head>

<body>

    <nav class="navbar navbar-inverse navbar-fixed-top ">
      <div class="container">
        <div class="navbar-header">
          <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
            <span class="sr-only">Toggle navigation</span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
          </button>
          <a class="navbar-brand " href="#">Project name</a>
        </div>
        <div id="navbar" class="collapse navbar-collapse">
          <ul class="nav navbar-nav navbar-right">
            <li class="active"><a href="#">Home</a></li>
            <li><a href="#about">About</a></li>
            <li><a href="#contact">Contact</a></li>
          </ul>
        </div><!--/.nav-collapse -->
      </div>
    </nav>

  
    
    <div class="container-fluid col-lg-12">

          <div class="col-xs-12 block">
              
                <div class="col-xs-3 block2 centerthis">
                </div>
              
          </div>
         
    </div><!-- /.container...