Question

The animation is supposed to swing back and forth. But I can’t figure out why it is not. Here is the code look at the website http://codepen.io/anon/pen/tasJE. That is that it is supposed to look like

CSS

#menu {
  position: relative;
  right: 30px;
  float: right;
  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: 71px;
  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: 0px 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: 200;
  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);
}

HTML

<ul id="menu">
  <li><a href="#">More</a>
        <ul>
            <li><a href="#">Home</a></li>
            <li><a href="#">Facts</a></li>
            <li><a href="#">Tour</a></li>
            <li><a href="#">Author</a></li>
        </ul>
</ul>​

And here is the jfiddle
http://jsfiddle.net/jspence29/CVnf4/

Answer 1

^{This is an extension to the comments above.}

JSFiddle allows you to load external JavaScript files, via the panel to the left.
That’s where I loaded -prefix-free into.

Here’s a screenshot: