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<nav>
  <div class="toggleNav">
    <div class="toggleNavButton"></div>
  </div>
  
  <div id="subnav">
    <ul>
      <li> <a href="#">Menu Item</a> </li>
      <li> <a href="#">Menu Item</a> </li>
      <li> <a href="#">Menu Item</a> </li>
      <li> <a href="#">Menu Item</a> </li>
    </ul>
  </div>
</nav>
*{
  -webkit-transition-timing-function: ease-out; /* Safari and Chrome */
  transition-timing-function: ease-out;
  box-sizing: border-box;
  text-decoration: none;
  list-style: none;
  padding: 0;
  margin: 0;
}

body{
  font-family: "Segoe UI", Frutiger, "Frutiger Linotype", "Dejavu Sans", "Helvetica Neue", Arial, sans-serif;
  color: #F2F2F2;
  background-color: #41558C;
}

.toggleNav{
  border-bottom: 5px solid darken(#54EFC9, 15%);
  background-color: #54EFC9;
  width: 100%;
  height: 80px;
  text-align: center;
  line-height: 80px;
  cursor: pointer;
  color: white;
  font-size: 30px;
  
  &:hover{
    border-bottom: 5px solid darken(#54EFC9, 20%);
  }
}

nav{
  width: 100%;
  min-height: 50px;
}

.toggleNavButton{
  transition-duration: 0.5s;
  width: 40px;
  height: 2px;
  background-color: white;
  position: absolute;
  left: 50%;
  top: 25px;
  margin-left: -20px;
  border-radius: 2px;
  
  &:before, &:after{
    border-radius: 2px;
    transition-duration: 0.5s;
    content: "";
    position: absolute;
    top: 10px;
    left: 0;
    background-color: white;
    width: 40px;
    height: 2px;
  }
  
  &:after{
    top: 20px;
  }
  
  &.active{
    width: 0;
    
    &:after{
      top: 10px;
      transform: rotate(45deg);
      -webkit-transform: rotate(45deg);
    }
    
    &:before{
      transform: rotate(-45deg);
      -webkit-transform: rotate(-45deg);
    }
  }
}

#subnav{
  width: 100%;
  border-bottom: 0px solid darken(#F64870, 15%);
  background-color: #F64870;
  transition-duration: 0.2s;
  overflow: hidden;
  height: 0px;
  
  &.active{
    border-bottom: 5px solid darken(#F64870, 15%);
    height: 50px;
  }
  
  ul{
    width: 615px;
    margin: 0 auto;
    
    li{
      transition-duration: 0.3s;
      display: inline-block;
      text-align: center;
      width: 150px;
      padding: 10px 0;
      height: 50px;
      
      a{
        color: white;
      }
      
      &:hover{
        background-color: darken(#F64870, 15%);
      }
    }
  }
}
View Compiled
$(".toggleNav").click(function () {
    $("#subnav").toggleClass("active");
    $(".toggleNavButton").toggleClass("active");
 });
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