CodePen

HTML

            
              
<div class="wrapper">
  <nav role='navigation'>
    <ul class="level-top">
      <li><a href="#">Home</a></li>
      <li class="has-children"><a href="#">About</a>
        <ul class="level-2">
          <li><a href="#">Item 1</a></li>
          <li><a href="#">Item 2</a></li>
        </ul>    
      </li>
      <li class="has-children"><a href="#">Clients</a>      
        <ul class="level-2"> 
          <li><a href="#">Item 1</a></li>
          <li><a href="#">Item 2</a></li>
          <li class="has-children"><a href="#">Item 3</a>
            <ul class="level-3">
              <li><a href="#">Item 3.1</a></li>
              <li><a href="#">Item 3.2</a></li>
              <li><a href="#">Item 3.3</a></li>
              <li><a href="#">Item 3.4</a></li>
            </ul>  
          </li>
          <li><a href="#">Item 4</a></li>
        </ul>  
      </li>
      <li><a href="#">Contact Us</a></li>
    </ul>
  </nav> 
  
  <div class="content">
    <p>Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Vestibulum tortor quam, feugiat vitae, ultricies eget, tempor sit amet, ante. Donec eu libero sit amet quam egestas semper. Aenean ultricies mi vitae est. Mauris placerat eleifend leo.</p>  
        <p>Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Vestibulum tortor quam, feugiat vitae, ultricies eget, tempor sit amet, ante. Donec eu libero sit amet quam egestas semper. Aenean ultricies mi vitae est. Mauris placerat eleifend leo.</p> 

  </div>
</div>
            
          
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CSS

            
              * {
  box-sizing:border-box;
}

.wrapper {
  margin:25px;
  background:AntiqueWhite ;
}

a {
  text-decoration:none;
  display:block;
  color:grey;
}

a:hover {
  color:white;
}

nav {
float:left;
}

/* TOP LEVEL STUFF */

.level-top {
  width:200px;
}

li {
  background: #292929;  
  border-top:1px solid black;
  border-right: 1px solid #3c3c3c;
  border-left: 1px solid #3c3c3c;
  position:relative;
}

li:last-child {
  border-bottom:1px solid black;
}

.has-children {
  /* no styles but this class would be probably be added via javascript */
}

.has-children:after {
  /* Adds sub-menu indicator */
  content:"";
  border-right:.5em solid transparent;
  border-left:.5em solid NavajoWhite;
  border-top:.5em solid transparent;
  border-bottom:.5em solid transparent;
  position:absolute;
  top:.5em;
  right:0;
  opacity:0.5;
}

.has-children:hover:after{
  border-left:.5em solid white;
  opacity:1;
}

/* LEVEL ONE/TWO/THREE STUFF */

.level-top > li > a,
.level-2 > li > a,
.level-3 > li > a {  
  padding-left:10px;
  font-size:16px;
  line-height:2em;
}

.level-2,
.level-3 {
  /* HIDES SUN-MENU UNTIL NEEDED */
  display:none;
}

.level-top li:hover .level-2,
.level-2 li:hover .level-3 {
  /* SHOWS SUB MENUES WHEN REQUIRED*/ 
  position:absolute;
  width:100%;
  display:block;
}

.level-2 > li,
.level-3 > li{
  /* POSITIONS SUB MENU LIST ITEMS */
  left:100%;
  top:-33px; /* = 16px + 16px (padding/line-height) +1px border */
}

.content {
  float:left;
  padding:10px;
  margin:10px;
  background:lightgrey;
}

.content p {
  padding: 1em 0;


            
          
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