Как создать подменю в раскрывающемся списке (HTML/CSS)

#menu {
  background: #343434;
  color: #eee;
  height: 35px;
  border-bottom: 4px solid #eeeded
}

#menu ul,
#menu li {
  margin: 0 0;
  padding: 0 0;
  list-style: none
}

#menu ul {
  height: 35px
}

#menu li {
  float: left;
  display: inline;
  position: relative;
  font: bold 12px Arial;
  text-shadow: 0 -1px 0 #000;
  border-right: 1px solid #444;
  border-left: 1px solid #111;
  text-transform: uppercase
}

#menu li:first-child {
  border-left: none
}

#menu a {
  display: block;
  line-height: 35px;
  padding: 0 14px;
  text-decoration: none;
  color: #eee;
}

#menu li:hover > a,
#menu li a:hover {
  background: #111
}

#menu input {
  display: none;
  margin: 0 0;
  padding: 0 0;
  width: 80px;
  height: 35px;
  opacity: 0;
  cursor: pointer
}

#menu label {
  font: bold 30px Arial;
  display: none;
  width: 35px;
  height: 36px;
  line-height: 36px;
  text-align: center
}

#menu label span {
  font-size: 12px;
  position: absolute;
  left: 35px
}

#menu ul.menus {
  height: auto;
  width: 180px;
  background: #111;
  position: absolute;
  z-index: 99;
  display: none;
  border: 0;
}

#menu ul.menus li {
  display: block;
  width: 100%;
  font: 12px Arial;
  text-transform: none;
}

#menu li:hover ul.menus {
  display: block
}

#menu a.home {
  background: #c00;
}

#menu a.prett {
  padding: 0 27px 0 14px
}

#menu a.prett::after {
  content: "";
  width: 0;
  height: 0;
  border-width: 6px 5px;
  border-style: solid;
  border-color: #eee transparent transparent transparent;
  position: absolute;
  top: 15px;
  right: 9px
}

#menu ul.menus a:hover {
  background: #333;
}

#menu ul.menus .submenu {
  display: none;
  position: absolute;
  left: 180px;
  background: #111;
  top: 0;
  width: 180px;
}

#menu ul.menus .submenu li {
  background: #111;
}

#menu ul.menus .has-submenu:hover .submenu {
  display: block;
}

<nav>
  <ul id='menu'>
    <li><a class='home' href='/'>Home</a></li>
    <li><a class='prett' href='#' title='Menu'>Menu</a>
      <ul class='menus'>
        <li class='has-submenu'><a class='prett' href='Dropdown 1' title='Dropdown 1'>Dropdown 1 + Sub Menu</a>
          <ul class='submenu'>
            <li><a href="#" title="Sub Menu">Sub Menu</a></li>
            <li><a href="#" title="Sub Menu">Sub Menu 2</a></li>
            <li><a href="#" title="Sub Menu">Sub Menu 3</a></li>
          </ul>
        </li>
        <li><a href='#' title='Dropdown 2'>Dropdown 2</a></li>
        <li><a href='#' title='Dropdown 3'>Dropdown 3</a></li>
      </ul>
    </li>
  </ul>
</nav>