JSFiddle - React, Tailwind, and code Playground

HTML

<div id="menu">
<ul id="nav">

<li><a href="#">SECTION 1</a>
 <ul>
  <li><a href="#">MENU 1</a></li>
  <li><a href="#">MENU 1</a></li>
  <li><a href="#">MENU 1</a></li>
  <li><a href="#">MENU 1</a></li>
 </ul>
</li> 

<li><a href="#">SECTION 2</a>
 <ul>
  <li><a href="#">MENU 2</a></li>
  <li><a href="#">MENU 2</a></li>
  <li><a href="#">MENU 2</a></li>
  <li><a href="#">MENU 2</a></li>
 </ul>
</li> 

<li><a href="#">SECTION 3</a>
 <ul>
  <li><a href="#">MENU 3</a></li>
  <li><a href="#">MENU 3</a></li>
  <li><a href="#">MENU 3</a></li>
  <li><a href="#">MENU 3</a></li>
 </ul>
</li> 


 </ul>

</div>

CSS

// css
#menu {
  width: 1002px;
  height: 40px;
  clear: both;
}

ul#nav {
  float: left;
  width: 1002px;
  margin: 0;
  padding: 0;
  list-style: none;
  background: #333 url(../img/menu-parent.png) repeat-x; 
}

ul#nav li {
  display: inline;
}

ul#nav li a {
  float: left;
  font: bold 0.6em arial,verdana,tahoma,sans-serif;
  line-height: 40px;
  color: #EEE;
  text-decoration: none;
  text-shadow: 1px 1px 1px #000;
  margin: 0;
  padding: 0 20px;
  background: #333 url(../img/menu-parent.png) repeat-x; 
}

@keyframes example {
    
    25% {
      display: block;
    opacity:.75
    }
        50% {
          display: block;
    opacity:.50
    }
            75% {
              display: block;
    opacity:.25
    }
    
            100% {display: block;
    opacity:0
    }
}
/* APPLIES THE ACTIVE STATE */
ul#nav .current a, ul#nav li:hover > a  {
  color: #58D3F7;
  text-decoration: none;
  text-shadow: 1px 1px 1px #000;
  font-weight: bold;
  background: #000;
}

/* THE SUBMENU LIST HIDDEN BY DEFAULT */
ul#nav  ul {
  display: none;
}

/* WHEN THE FIRST LEVEL MENU ITEM IS HOVERED, THE CHILD MENU APPEARS */
ul#nav li:hover > ul {
  position: absolute;
  display: block;
  width: 962px;
  height: auto;
  position: absolute;
  margin: 40px 0 0 0;
  background: #58D3F7 url(../img/menu-child.png) repeat-x; 
  z-index: 9999; 
  animation-name: example;
    animation-duration: 250ms;
    animation-iteration-count:1;
    animation-direction: reverse;
}

ul#nav li:hover > ul li a {
  float: left;
  font: bold 0.7em arial,verdana,tahoma,sans-serif;
  line-height: 45px;
  width: 200px;
  color: #000;
  text-decoration: none;
  text-shadow: none;
  margin: 0;
  padding: 0 30px 0 0;
  background: #58D3F7 url(../img/menu-child.png) repeat-x; 
}

ul#nav li:hover > ul li a:hover {
  color: #FFF;
  text-decoration: none;
  text-shadow: none;
  font-weight: bold;
}