JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main-nav">
<ul>
    <li><a href="http://mysite.com">Item 1</a></li>
    <li><a href="http://mysite.com">Item 2</a></li>
    <li><a href="http://mysite.com">Item 3</a>
        <!-- ///////////////////////// Start Blog Drop Down ///////////////////////// -->
        <div class="nav-dropdown">
            <p>have this item slide down from CSS</p>
        </div>
        <!-- /.nav-dropdown -->
    </li>
</ul>
</div>

CSS

#main-nav {
	position: relative;
}

#main-nav > ul > li {
	width: 12.5%;
	float: left;
	text-align: center;
	line-height: .9em;
	font-weight: bold;
	background: #ccc;
}

#main-nav > ul > li > a {
	display: block;
	color: #333;
	line-height: 12px;
	font-size: 14px;
	padding: 22px 0;
	text-decoration: none;
}

.nav-dropdown {
    margin: 0 auto;  
    height: 0; /* Hides the drop down */ 
    overflow: hidden;
    text-align:left;  
    padding: 0;  
    border-top: 0;
    width: 500px;
    background:#333;
    color: #f2f2f2;
    -webkit-border-bottom-right-radius: 5px;
	-webkit-border-bottom-left-radius: 5px;
	-moz-border-radius-bottomright: 5px;
	-moz-border-radius-bottomleft: 5px;
	border-bottom-right-radius: 5px;
	border-bottom-left-radius: 5px;
    border-bottom: 0;
    max-height:0px;
}

#main-nav li:hover .nav-dropdown {
    height:auto;
    z-index: 11; 
    max-height:200px;
	transition-property: all;
	transition-duration: 1s;
    border-bottom: 10px solid #25272a;
}