* {
    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;
  }
  
  .menu li {
    float: left;
    width: 25%;
    height: 100%;
    margin: 0;
    padding: 0;
  }
  
  .menu 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 ~ .slider {
    left: 0;
    background-color: #3498db;
  }
  
  .menu li:nth-child(2):hover ~ .slider {
    left: 25%;
    background-color: #9b59b6;
  }
  
  .menu li:nth-child(3):hover ~ .slider {
    left: 50%;
    background-color: #e67e22;
  }
  
  .menu li:nth-child(4):hover ~ .slider {
    left: 75%;
    background-color: #16a085;
  }