Pure CSS Dropdown Menu with 3D Flipping Animation

A pure CSS dropdown menu that reveals the sub menu with a 3D flipping animation when you hover over a parent menu.

How to use it:

Create the Html for a 2-level dropdown menu using nested Html lists.

<ul class="menu">
  <li class="parent"> <a href="">Parent</a>
    <ul class="children">
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
    </ul>
  </li>
  <li class="parent"> <a href="">Parent</a>
    <ul class="children">
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
    </ul>
  </li>
  <li class="parent"> <a href="">Parent</a>
    <ul class="children">
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
      <li><a href="">Item</a></li>
    </ul>
  </li>
</ul>

The basic CSS styles for the dropdown menu.

.menu {
  display: inline-block;
  margin: 0 auto;
  margin-top: 100px;
  list-style-type: none;
}
.menu a {
  display: block;
  margin: 0;
  padding: 16px 25px;
  color: #333;
  text-decoration: none;
  background-color: #fff;
}
.menu li {
  position: relative;
  float: left;
  margin: 0;
  border-left: 1px solid #eee;
  -webkit-perspective: 200;
  perspective: 200;
}
.menu li:first-child { border-left: none; }
.menu li.parent:before {
  content: '';
  z-index: 200;
  position: absolute;
  top: 100%;
  left: 50%;
  margin-top: -4px;
  margin-left: -20px;
  display: block;
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 8px 20px 0 20px;
  border-color: #fff transparent transparent transparent;
  -webkit-transition: margin-top .1s ease-out;
  transition: margin-top .1s ease-out;
}
.menu li:hover > a,
.menu li:focus > a {
  text-decoration: none;
  color: #fff;
  background-color: #333;
}
.menu li:hover:before,
.menu li:focus:before {
  margin-top: 0;
  border-top-color: #333;
}

The core CSS styles to create the 3D flipping effect using CSS3 transforms and transitions.

.menu li:hover .children,
.menu li:focus .children {
  opacity: 1;
  -webkit-transform: rotateX(0) translateZ(0);
  transform: rotateX(0) translateZ(0);
}
.menu .children {
  opacity: 0;
  -webkit-backface-visibility: hidden;
  backface-visibility: hidden;
  -webkit-transform: rotateX(-90deg);
  transform: rotateX(-90deg);
  -webkit-transform-style: preserve-3d;
  transform-style: preserve-3d;
  -webkit-transform-origin: top center;
  -ms-transform-origin: top center;
  transform-origin: top center;
  -webkit-transition: -webkit-transform 0.4s cubic-bezier(0.17, 0.67, 0.59, 1.21), opacity 0.1s 0.1s;
  transition: transform 0.4s cubic-bezier(0.17, 0.67, 0.59, 1.21), opacity 0.1s 0.1s;
  z-index: 100;
  list-style-type: none;
  position: absolute;
  top: 100%;
  left: 0;
  width: 200px;
  margin: 0;
  padding: 10px 0;
  background-color: #fff;
  box-shadow: 0 2px 3px 0 rgba(0, 0, 0, 0.1);
  text-align: left;
}
.menu .children li { float: none; }
.menu .children a { background-color: transparent; }
.menu .children a:hover,
.menu .children a:focus {
  color: #333;
  background-color: #f9f9f9;
}