JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" rel="stylesheet"/>

<nav class="navbar navbar-inverse navbar-fixed-top">
  <div class="container-fluid">
    <div class="navbar-header">
      <a class="navbar-brand" href="#">WebSiteName</a>
    </div>
    <ul class="nav navbar-nav">
      <li class="active"><a href="#">Home</a></li>
      <li><a href="#">Page 1</a></li>
      <li><a href="#">Page 2</a></li> 
      <li><a href="#">Page 3</a></li> 
    </ul>
  </div>
</nav>

<div class="container">                
                  
    
    <div>                                                    
        
        <div>Box 1</div>      
        <div>                           
        
            <div>Box 2</div>      
            
            <div>Box 3</div>       

        </div>  
        
        
    </div>                             
    
  <div>Box 4</div>                     
  
</div>

CSS

html, body, .container {
    height: 100%;
    background-color: white;
}

.container {
    display: flex;
    flex-direction: column; 
    margin-top: 50px;
}

.container div {                   
    margin: 10px;
    flex: 1;
    background-color: blue;
}

.container > div:nth-child(2) { }

.container > div:first-child {
    display: flex;
    background-color: white;
    
}
.container > div:first-child > div:first-child {
    margin-right: 20px;
    
}

.container > div:first-child > div:nth-child(2) {
    display: flex;
    flex-direction: column;
    background-color: white;
    
}

.container > div:first-child div {
    flex: 1;
    margin: 0;
}

.container > div:first-child > div:nth-child(2) > div:first-child {
    margin-bottom: 20px;
}

.container > div:last-child { }