<h1>Responsive Menu Effect
hover on menu items!
</h1>

<ul class="menu">
  <li><a href="#" class="active">Home</a></li>
  <li><a href="#">About</a></li>
  <li><a href="#">Portfolio</a></li>
  <li><a href="#">Contact</a></li>
  <li class="slider"></li>
</ul>

* {
  box-sizing: border-box;
}

@import url(https://fonts.googleapis.com/css?family=PT+Sans);

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;

  li {
    float: left;
    width: 25%;
    height: 100%;
    margin: 0;
    padding: 0;
  }
  
  a {
    display: flex;
    width: 100%;
    height: 100%;
    justify-content: 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;
  transition: left .4s, background-color .4s;
}

.menu li:nth-child(1):hover,
.menu li:nth-child(1):focus,
.menu li:nth-child(1):active {
  & ~ .slider {
    left: 0;
    background-color: #3498db;
  }
}


.menu li:nth-child(2):hover,
.menu li:nth-child(2):focus,
.menu li:nth-child(2):active {
  & ~ .slider {
    left: 25%;
    background-color: #9b59b6;
  }
}

.menu li:nth-child(3):hover,
.menu li:nth-child(3):focus,
.menu li:nth-child(3):active {
  & ~ .slider {
    left: 50%;
    background-color: #e67e22;
  }
}

.menu li:nth-child(4):hover,
.menu li:nth-child(4):focus,
.menu li:nth-child(4):active {
  & ~ .slider {
    left: 75%;
    background-color: #16a085;
  }
}