<ul class="hList">
  <li>
    <details class="dropdown">
      <summary>animals</summary>
      <ul class="menu-dropdown">
        <li><a href="##">acat</a></li>
        <li><a href="##">adog</a></li>
        <li><a href="##">ahorse</a></li>
        <li><a href="##">acow</a></li>
        <li><a href="##">pig</a></li>
      </ul>
    </details>
  </li>
  <li>
    <details class="dropdown">
      <summary>names</summary>
      <ul class="menu-dropdown">
        <li><a href="##">Kevin</a></li>
        <li><a href="##">Jim</a></li>
        <li><a href="##">Andy</a></li>
      </ul>
    </details>
  </li>
  <li>
    <details class="dropdown">
      <summary>things</summary>
      <ul class="menu-dropdown">
        <li><a href="##">bench</a></li>
        <li><a href="##">pizza</a></li>
        <li><a href="##">Honda CB125</a></li>
        <li><a href="##">space</a></li>
        <li><a href="##">black matter</a></li>
        <li><a href="##">apple</a></li>
        <li><a href="##">philodendron</a></li>
        <li><a href="##">liver</a></li>
        <li><a href="##">brass</a></li>
      </ul>
    </details>
  </li>
  <li>
    <details class="dropdown">
      <summary>Movies</summary>
      <ul class="menu-dropdown">
        <li><a href="##">Godzilla</a></li>
        <li><a href="##">Man on Wire</a></li>
        <li><a href="##">Spirited Away</a></li>
        <li><a href="##">Interstellar</a></li>
      </ul>
    </details>
  </li>
</ul>  
@import url(https://fonts.googleapis.com/css?family=Roboto:400,700);

html {
  height: 100%;
  background-color: #f8f8f8;
}

body {
  overflow: hidden;
  height: 100%;
  width: 600px;
  margin: 0 auto;
  background-color: #ffffff;
  font-family: 'Roboto', sans-serif;
  color: #555555;
}

a {
  text-decoration: none;
  color: inherit;
  display: block;
}

* {
  box-sizing: border-box;
}

$menu_WIDTH: 150px;

ul, li {
  list-style: none outside none;
  margin: 0;
  padding: 0;
}
.dropdown summary { 
  direction: rtl; 
} 
.dropdown ::-webkit-details-marker { 
  direction: ltr; 
  margin-left: 1ch; 
} 
.dropdown ::-moz-list-bullet { 
  direction: ltr;
  margin-left: 1ch; 
}

.dropdown {
  display: block;
  position: relative;
  cursor: pointer;
}

summary {
  display: block;
  width: $menu_WIDTH;
  height: 40px;
  padding: 12px 0 0;
  background: #9dc852;
  text-align: center;
  color: #ffffff;
  font-weight: bold;
  text-transform: uppercase;
  transition: 0.3s background-color;
}

summary:before {
  content: "";
  display: block;
  height: 0;
  border-top: 5px solid #9dc852;
  border-left: ($menu_WIDTH / 2) solid transparent;
  border-right: ($menu_WIDTH / 2) solid transparent;
  border-bottom: 0 solid #dddddd;
  position: absolute;
  top: 100%;
  left: 0;
  z-index: 101;
  transition:
    0.2s 0.2s border-top ease-out,
    0.3s border-top-color;
}

.summary:hover { background: #8db842; }
summary:hover:before { border-top-color: #8db842; }

.dropdown:hover > summary:before {
  border-top-width: 0;
  transition:
    0.2s border-top-width ease-in,
    0.3s border-top-color;
}

summary:after {
  content: "";
  display: block;
  height: 0;
  border-left: ($menu_WIDTH / 2) solid transparent;
  border-right: ($menu_WIDTH / 2) solid transparent;
  border-bottom: 0 solid #ebebeb;
  position: absolute;
  bottom: 0;
  left: 0;
  z-index: 101;
  transition: 0.2s border-bottom ease-in;
}

.dropdown:hover > summary:after {
  border-bottom-width: 5px;
  transition: 0.2s 0.2s border-bottom-width ease-out;
}

li:nth-child(2) summary { background: #4e96b3; }
li:nth-child(2) summary:hover { background: #3e86a3; }
li:nth-child(2) summary:before { border-top-color: #4e96b3; }
li:nth-child(2) summary:hover:before { border-top-color: #3e86a3; }

li:nth-child(3) summary { background: #c97676; }
li:nth-child(3) summary:hover { background: #b96666; }
li:nth-child(3) summary:before { border-top-color: #c97676; }
li:nth-child(3) summary:hover:before { border-top-color: #b96666; }

li:nth-child(4) summary { background: #dbab58; }
li:nth-child(4) summary:hover { background: #cb9b48; }
li:nth-child(4) summary:before { border-top-color: #dbab58; }
li:nth-child(4) summary:hover:before { border-top-color: #cb9b48; }

.menu-dropdown {
  min-width: 100%;
  padding: 15px 0;
  position: absolute;
  background: #ebebeb;
  z-index: 100;
  transition:
    0.5s padding,
    0.5s background;
}

.menu-dropdown:after {
  content: "";
  display: block;
  height: 0;
  border-top: 5px solid #ebebeb;
  border-left: ($menu_WIDTH / 2) solid transparent;
  border-right: ($menu_WIDTH / 2) solid transparent;
  position: absolute;
  top: 100%;
  left: 0;
  z-index: 101;
  transition: 0.5s border-top;
}

.dropdown:not(:hover) > .menu-dropdown {
  padding: 4px 0;
  background: #dddddd;
  z-index: 99;
}

.dropdown:not(:hover) > .menu-dropdown:after {
  border-top-color: #dddddd;
}

.dropdown:not(:hover) > summary:after {
  border-bottom-color: #dddddd;
}

.dropdown:focus,
.dropdown:active {
  outline: none;
}
.menu-dropdown > * {
  overflow: hidden;
  height: 40px;
  line-height: 40px;
  padding: 5px 10px;
  background: rgba(0,0,0,0);
  white-space: nowrap;
  transition: 
    0.5s height cubic-bezier(.73,.32,.34,1.5),
    0.5s padding cubic-bezier(.73,.32,.34,1.5),
    0.5s margin cubic-bezier(.73,.32,.34,1.5),
    0.5s 0.2s color,
    0.2s background-color;
}

.menu-dropdown > *:hover {
  background: rgba(0,0,0,0.1);
}

.dropdown:not(:hover) > .menu-dropdown > * {
  padding-top: 0;
  padding-bottom: 0;
  margin: 0;
  transition: 
    0.5s 0.1s height,
    0.5s 0.1s padding,
    0.5s 0.1s margin,
    0.3s color,
    0.6s visibility;
  z-index: 99;
}

.hList {
  //overflow: hidden;
}

.hList > * {
  float: left;
}

.hList > * + * {
  margin-left: 0;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.