Sliding Menu Tabs

by panchroma

HTML

<ul class="menu">
  <li><a href="#" class="active">Option1</a></li>
  <li><a href="#">Option2</a></li>
  <li><a href="#">Option3</a></li>
  <li><a href="#">Option4</a></li>
  <li class="slider"></li>
</ul>

CSS

@import url(http://fonts.googleapis.com/css?family=PT+Sans);
* {
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}
html,
body {
  margin: 0;
  padding: 0;
}
body {
  font-family: 'PT Sans', Arial, Verdana;
  background-color: #eee;
}
h1 {
  text-align: center;
  font-size: 48px;
  text-transform: uppercase;
  letter-spacing: 3px;
  color: #222;
}
.menu {
  list-style: none;
  margin: 0;
  padding: 0;
  width: 100%;
  height: 120px;
  margin: auto;
  position: relative;
  background-color: #2c3e50;
  z-index: 7;
}
.menu li {
  float: left;
  width: 25%;
  height: 100%;
  margin: 0;
  padding: 0;
}
.menu a {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  width: 100%;
  height: 100%;
  -webkit-box-pack: center;
  -webkit-justify-content: center;
  -ms-flex-pack: center;
  justify-content: center;
  -webkit-box-align: center;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
  color: #fff;
  text-decoration: none;
  position: relative;
  font-size: 18px;
  z-index: 9;
}
a.active {
  background-color: #e74c3c;
  pointer-events: none;
}
li.slider {
  width: 25%;
  height: 100%;
  position: absolute;
  left: 0;
  top: 0;
  background-color: #e74c3c;
  z-index: 8;
  -webkit-transition: left 0.4s, background-color 0.4s;
  transition: left 0.4s, background-color 0.4s;
}
.menu li:nth-child(1):hover ~ .slider,
.menu li:nth-child(1):focus ~ .slider,
.menu li:nth-child(1):active ~ .slider {
  left: 0;
  background-color: #3498db;
}
.menu li:nth-child(2):hover ~ .slider,
.menu li:nth-child(2):focus ~ .slider,
.menu li:nth-child(2):active ~ .slider {
  left: 25%;
  background-color: purple;
}
.menu li:nth-child(3):hover ~ .slider,
.menu li:nth-child(3):focus ~ .slider,
.menu li:nth-child(3):active ~ .slider {
  left: 50%;
  background-color: #e67e22;
}
.menu li:nth-child(4):hover ~ .slider,
.menu li:nth-child(4):focus ~ .slider,
.menu li:nth-child(4):active ~...