Simple Accordion Menu with CSS3 Transitions

A simple accordion navigation menu built with only CSS and animated by CSS3 transitions.

How to use it:

Build the structure for an accordion menu using nested Html unordered lists.

<nav id="menu_box">
  <ul class="menu">
    <li> <a href="#">Menu 1</a>
      <ul>
        <li><a href="#">Menu 1.1</a></li>
        <li><a href="#">Menu 1.2</a></li>
      </ul>
    </li>
    <li> <a href="#">Menu 2</a>
      <ul>
        <li><a href="#">Menu 2.1</a></li>
        <li><a href="#">Menu 2.2</a></li>
        <li><a href="#">Menu 2.3</a></li>
      </ul>
    </li>
    <li> <a href="#">Menu 3</a>
      <ul>
        <li><a href="#">Menu 3.1</a></li>
        <li><a href="#">Menu 3.2</a></li>
        <li><a href="#">Menu 3.3</a></li>
        <li><a href="#">Menu 3.4</a></li>
      </ul>
    </li>
    <li> <a href="#">Menu 4</a> </li>
  </ul>
</nav>

The required CSS styles for the accordion menu.

.menu {
  margin: 0 auto;
  padding: 0;
  width: 350px;
}
.menu li { list-style: none; }
.menu li a {
  display: table;
  margin-top: 1px;
  padding: 14px 10px;
  width: 100%;
  background: #337D88;
  text-decoration: none;
  text-align: left;
  vertical-align: middle;
  color: #fff;
  overflow: hidden;
  -webkit-transition-property: background;
  -webkit-transition-duration: 0.4s;
  -webkit-transition-timing-function: ease-out;
  transition-property: background;
  transition-duration: 0.4s;
  transition-timing-function: ease-out;
}
.menu > li:first-child a { margin-top: 0; }
.menu li a:hover {
  background: #4AADBB;
  -webkit-transition-property: background;
  -webkit-transition-duration: 0.2s;
  -webkit-transition-timing-function: ease-out;
  transition-property: background;
  transition-duration: 0.2s;
  transition-timing-function: ease-out;
}
.menu li ul {
  margin: 0;
  padding: 0;
}
.menu li li a {
  display: block;
  margin-top: 0;
  padding: 0 10px;
  height: 0;
  background: #C6DDD9;
  color: #1F3D39;
  -webkit-transition-property: all;
  -webkit-transition-duration: 0.5s;
  -webkit-transition-timing-function: ease-out;
  transition-property: all;
  transition-duration: 0.5s;
  transition-timing-function: ease-out;
}
.menu > li:hover li a {
  display: table;
  margin-top: 1px;
  padding: 10px;
  width: 100%;
  height: 1em;
  -webkit-transition-property: all;
  -webkit-transition-duration: 0.3s;
  -webkit-transition-timing-function: ease-out;
  transition-property: all;
  transition-duration: 0.3s;
  transition-timing-function: ease-out;
}
.menu > li:hover li a:hover {
  background: #A4CAC8;
  -webkit-transition-property: background;
  -webkit-transition-duration: 0.2s;
  -webkit-transition-timing-function: ease-out;
  transition-property: background;
  transition-duration: 0.2s;
  transition-timing-function: ease-out;
}