Nested Menu CSS Hover Fade

Question

I modified this menu and added css opacity hover transition.

It only works on the first ul, but the nested sub ul's will not show.
https://jsfiddle.net/sh73vvgf/

Replacing the fade effect with display: none; will show the sub menus.
https://jsfiddle.net/v8op2tqv/

How can I get fade to work on the sub menus? I need it to be pure html/css.

HTML

This nested menu shows a list of Planets and their Moons.

<div class="nav-main">
  <ul>
    <li>
      Planets
      <ul>
        <li>
          Jupiter
          <!-- Moons -->
          <ul>
            <li>
              Europa
            </li>
            <li>
              Ganymede
            </li>
            <li>
              Callisto
            </li>
          </ul>
        </li>
        <li>
          Saturn
          <!-- Moons -->
          <ul>
            <li>
              Mimas
            </li>
            <li>
              Dione
            </li>
            <li>
              Titan
            </li>
          </ul>
        </li>
        <li>
          Uranus
          <!-- Moons -->
          <ul>
            <li>
              Umbriel
            </li>
            <li>
              Ariel
            </li>
            <li>
              Oberon
            </li>
          </ul>
        </li>
      </ul>
    </li>
  </ul>
</div>

CSS

.nav-main ul {
  cursor: default;
  font-family: "Lucida Sans Unicode", "Lucida Grande", sans-serif;
  position: relative;
  width: 300px;
  padding: 0;
  margin: 0;
  background: #000;
  font-size: 1em;
  color: #fff;
  list-style: none;
}

.nav-main ul li {
  display: block;
  position: relative;
  float: left;
  min-width: 25%;
  padding: 0.3em;
  min-width: 100px;
}

.nav-main ul li a {
  display: block;
  padding: 1.2em;
  text-decoration: none;
  text-align: center;
  white-space: nowrap;
  color: #fff;
}

/* Hover Fade */
.nav-main li ul { 
  webkit-transition: opacity 0.3s ease-out, -webkit-transform 0.3s ease-out;
  -moz-transition: opacity 0.3s ease-out, -moz-transform 0.3s ease-out;
  transition: opacity 0.3s ease-out, transform 0.3s ease-out;
  opacity: 0;
  height: 0;
  width: 0;
  overflow: hidden;
}
.nav-main li:hover > ul {
  display: block;
  position: absolute;
  opacity: 1;
  height: auto;
  width: auto;
}

.nav-main li:hover li { 
    float: none; 
}

.nav-main ul ul ul {
  left: 100%;
  top: 0;
}

.nav-main ul:before,
.nav-main ul:after {
  content: " "; /* 1 */
  display: table; /* 2 */
}

.nav-main ul:after { 
    clear: both; 
}

Answer 1

I changed some "overflow" style of your code.

.nav-main ul {
  cursor: default;
  font-family: "Lucida Sans Unicode", "Lucida Grande", sans-serif;
  position: relative;
  width: 300px;
  padding: 0;
  margin: 0;
  background: #000;
  font-size: 1em;
  color: #fff;
  list-style: none;
  overflow:hidden;
}
.nav-main ul:hover {
  overflow:visible;
}

.nav-main ul li {
  display: block;
  position: relative;
  float: left;
  min-width: 25%;
  padding: 0.3em;
  min-width: 100px;
}

.nav-main ul li a {
  display: block;
  padding: 1.2em;
  text-decoration: none;
  text-align: center;
  white-space: nowrap;
  color: #fff;
}

/* Hover Fade */
.nav-main li ul { 
  webkit-transition: opacity 0.3s ease-out, -webkit-transform 0.3s ease-out;
  -moz-transition: opacity 0.3s ease-out, -moz-transform 0.3s ease-out;
  transition: opacity 0.3s ease-out, transform 0.3s ease-out;
  opacity: 0;
  height: 0;
  width: 0;
  //overflow: hidden;
}
.nav-main li:hover > ul {
  display: block;
  position: absolute;
  opacity: 1;
  height: auto;
  width: auto;
}

.nav-main li:hover li { 
    float: none; 
}

.nav-main ul ul ul {
  left: 100%;
  top: 0;
}

.nav-main ul:before,
.nav-main ul:after {
  content: " "; /* 1 */
  display: table; /* 2 */
}

.nav-main ul:after { 
    clear: both; 
}

<div class="nav-main">
  <ul>
    <li>
      Planets
      <ul>
        <li>
          Jupiter
          <!-- Moons -->
          <ul>
            <li>
              Europa
            </li>
            <li>
              Ganymede
            </li>
            <li>
              Callisto
            </li>
          </ul>
        </li>
        <li>
          Saturn
          <!-- Moons -->
          <ul>
            <li>
              Mimas
            </li>
            <li>
              Dione
            </li>
            <li>
              Titan
            </li>
          </ul>
        </li>
        <li>
          Uranus
          <!-- Moons -->
          <ul>
            <li>
              Umbriel
            </li>
            <li>
              Ariel
            </li>
            <li>
              Oberon
            </li>
          </ul>
        </li>
      </ul>
    </li>
  </ul>
</div>