A CodePen by Seth Abbott. CSS Swinging Panel Menu - A simple, clean pure CSS menu that uses keyframe animation to create a "swining panel" effect for sub nav. I am having trouble getting it to work in Opera but performs well in Chrome, Safari and Firefox

index.html

<div id="container">
<ul id="menu">
  <li><a href="#">About Me</a>
		<ul>
			<li><a href="#">Lorem ipsum dolor</a></li>
			<li><a href="#">Maecenas lacinia sem</a></li>
			<li><a href="#">Suspendisse fringilla</a></li>
		</ul>
	</li>
	<li><a href="#">Portfolio</a>
		<ul>
			<li><a href="#">Lorem ipsum dolor</a></li>
			<li><a href="#">Maecenas dignissim fermentum luctus</a></li>
			<li><a href="#">Suspendisse fringilla</a></li>
			<li><a href="#">Lorem ipsum dolor</a></li>
			<li><a href="#">Maecenas lacinia sem</a></li>
			<li><a href="#">Suspendisse fringilla</a></li>
		</ul>
	</li>
	<li><a href="#">Clients</a>
		<ul>
			<li><a href="#">Lorem ipsum dolor</a></li>
			<li><a href="#">Maecenas lacinia sem</a></li>
			<li><a href="#">Suspendisse fringilla</a></li>
		</ul>
	</li>
	<li><a href="#">Contact Me</a>
		<ul>
			<li><a href="#">Lorem ipsum dolor</a></li>
			<li><a href="#">Maecenas dignissim fermentum luctus</a></li>
			<li><a href="#">Suspendisse fringilla</a></li>
		</ul>
	</li>
	<li><a href="#">Support</a></li>
</ul>
</div>

style.css

* {
  	margin: 0; 
		padding: 0;
		box-sizing: border-box;
	}

	body {
		padding: 30px; 
		font-family: "Helvetica Neue", helvetica, arial; 
		background: url('http://subtlepatterns.com/patterns/white_carbonfiber.png');
	}

	#container {
		position: relative;
		width: 940px;		
	}

	#container:after {
		content: "";
		display: block;
		clear: both;
		height: 0;
	}

	#menu {
		position: relative;
		float: left;
		width: 100%;
		padding: 0 20px;
		border-radius: 3px;
		box-shadow: inset 0 1px 1px rgba(255,255,255,.5), inset 0 -1px 0 rgba(0,0,0,.15), 0 1px 3px rgba(0,0,0,.15);
		background: #ccc; 
	}

	#menu, #menu ul {
		list-style: none;
	}

	#menu > li {
		float: left;
		position: relative;
		border-right: 1px solid rgba(0,0,0,.1);
		box-shadow: 1px 0 0 rgba(255,255,255,.25);
		perspective: 1000px;
		
	}

	#menu > li:first-child {
		border-left: 1px solid rgba(255,255,255,.25);
		box-shadow: -1px 0 0 rgba(0,0,0,.1), 1px 0 0 rgba(255,255,255,.25);
	}

	#menu a {
		display: block;
		position: relative;
		z-index: 10;
		padding: 13px 20px 13px 20px;
		text-decoration: none;
		color: rgba(75,75,75,1);
		line-height: 1;
		font-weight: 600;
		font-size: 12px;
		letter-spacing: -.05em;
		background: transparent;		
		text-shadow: 0 1px 1px rgba(255,255,255,.9);
		transition: all .25s ease-in-out;
	
	}

	#menu > li:hover > a {
		background: #333;
		color: rgba(0,223,252,1);
		text-shadow: none;
	}

	#menu li ul  {
		position: absolute;
		left: 0;
		z-index: 1;
		width: 200px;
		padding: 0;
		opacity: 0;
		visibility: hidden;
		border-bottom-left-radius: 4px;
		border-bottom-right-radius: 4px;
		background: transparent;
		overflow: hidden;
		transform-origin: 50% 0%;
	}


	#menu li:hover ul {
		
		padding: 15px 0;
		background: #333;
		opacity: 1;
		visibility: visible;
		box-shadow: 1px 1px 7px rgba(0,0,0,.5);
		animation-name: swingdown;
		animation-duration: 1s;
		animation-timing-function: ease;

	}

	@keyframes swingdown {
		0% {
			opacity: .99999;
			transform: rotateX(90deg);
		}

		30% {			
			transform: rotateX(-20deg) rotateY(5deg);
			animation-timing-function: ease-in-out;
		}

		65% {
			transform: rotateX(20deg) rotateY(-3deg);
			animation-timing-function: ease-in-out;
		}

		100% {
			transform: rotateX(0);
			animation-timing-function: ease-in-out;
		}
	}

	#menu li li a {
		padding-left: 15px;
		font-weight: 400;
		color: #ddd;
		text-shadow: none;
		border-top: dotted 1px transparent;
		border-bottom: dotted 1px transparent;
		transition: all .15s linear;
	}

	#menu li li a:hover {
		color: rgba(0,223,252,1);
		border-top: dotted 1px rgba(255,255,255,.15);
		border-bottom: dotted 1px rgba(255,255,255,.15);
		background: rgba(0,223,252,.02);
	}