<ul class="menu cf">
  <li><a href="">Menu item</a></li>
  <li>
    <a href="">Menu item</a>
    <ul class="submenu">
      <li><a href="">Submenu item</a></li>
      <li><a href="">Submenu item</a></li>
      <li><a href="">Submenu item</a></li>
      <li><a href="">Submenu item</a></li>
    </ul>     
  </li>
  <li><a href="">Menu item</a></li>
  <li><a href="">Menu item</a></li>
  <li><a href="">Menu item</a></li>
</ul>

/* Clearing floats */
.cf:before,
.cf:after {
  content: " ";
  display: table;
}

.cf:after {
  clear: both;
}

.cf {
  *zoom: 1;
}

/* Mini reset, no margins, paddings or bullets */
.menu,
.submenu {
  margin: 0;
  padding: 0;
  list-style: none;
}

/* Main level */
.menu {     
  margin: 50px auto;
  width: 800px;
  /* http://www.red-team-design.com/horizontal-centering-using-css-fit-content-value */
  width: -moz-fit-content;
  width: -webkit-fit-content;
  width: fit-content; 
}

.menu > li {
  background: #34495e;
  float: left;
  position: relative;
  transform: skewX(25deg);
}

.menu a {
  display: block;
  color: #fff;
  text-transform: uppercase;
  text-decoration: none;
  font-family: Arial, Helvetica;
  font-size: 14px;
}   

.menu li:hover {
  background: #e74c3c;
}   

.menu > li > a {
  transform: skewX(-25deg);
  padding: 1em 2em;
}

/* Dropdown */
.submenu {
  position: absolute;
  width: 200px;
  left: 50%; margin-left: -100px;
  transform: skewX(-25deg);
  transform-origin: left top;
}

.submenu li {
  background-color: #34495e;
  position: relative;
  overflow: hidden;   
}           

.submenu > li > a {
  padding: 1em 2em;     
}

.submenu > li::after {
  content: '';
  position: absolute;
  top: -125%;
  height: 100%;
  width: 100%;      
  box-shadow: 0 0 50px rgba(0, 0, 0, .9);     
}   

/* Odd stuff */
.submenu > li:nth-child(odd){
  transform: skewX(-25deg) translateX(0);
}

.submenu > li:nth-child(odd) > a {
  transform: skewX(25deg);
}

.submenu > li:nth-child(odd)::after {
  right: -50%;
  transform: skewX(-25deg) rotate(3deg);
}       

/* Even stuff */
.submenu > li:nth-child(even){
  transform: skewX(25deg) translateX(0);
}

.submenu > li:nth-child(even) > a {
  transform: skewX(-25deg);
}

.submenu > li:nth-child(even)::after {
  left: -50%;
  transform: skewX(25deg) rotate(3deg);
}

/* Show dropdown */
.submenu,
.submenu li {
  opacity: 0;
  visibility: hidden;     
}

.submenu li {
  transition: .2s ease transform;
}

.menu > li:hover .submenu,
.menu > li:hover .submenu li {
  opacity: 1;
  visibility: visible;
}   

.menu > li:hover .submenu li:nth-child(even){
  transform: skewX(25deg) translateX(15px);     
}

.menu > li:hover .submenu li:nth-child(odd){
  transform: skewX(-25deg) translateX(-15px);     
}

// https://catalin.red/dropdown-menu-concept/