Transition d'animation CSS3: l'opacité ne fonctionne pas

je vais avoir quelques problèmes avec l'animation d'un menu déroulant à l'aide de css3. J'en ai besoin pour travailler avec css3 et pas jQuery ou javascript. J'ai ajouté toutes les règles nécessaires, mais encore l'effet ne se produit pas. Quelqu'un peut-il aider s'il vous plaît? Voici un violon de mon code. Merci.

http://jsfiddle.net/Zmr7u/6/

   html code:                 
<header class="main-header">
<nav class="main-nav">
<ul class="top-nav">
<li>
<a href="#">home</a>
<ul class="drop-down">
<li><a href="#">1</a></li>
<li><a href="#">2</a></li>
<li><a href="#">3</a></li>
</ul>
</li>
<li>
<a href="#">about</a>
<ul class="drop-down">
<li><a href="#">1</a></li>
<li><a href="#">2</a></li>
<li><a href="#">3</a></li>
</ul>
</li>
<li>
<a href="#">products</a>
<ul class="drop-down">
<li><a href="#">1</a></li>
<li><a href="#">2</a></li>
<li><a href="#">3</a></li>
</ul>
</li>
<li>
<a href="#">contacts</a>
<ul class="drop-down">
<li><a href="#">1</a></li>
<li><a href="#">2</a></li>
<li><a href="#">3</a></li>
</ul>
</li>
</ul>
</nav>
</header>
css code:
nav.main-nav {
background: #333;
background-image: -webkit-gradient(linear, 0% 0%, 0% 100%, from(#2f2f2f), to(#4f4f4f));
background-image: -webkit-linear-gradient(top, #2f2f2f, #4f4f4f); 
background-image:    -moz-linear-gradient(top, #2f2f2f, #4f4f4f);
background-image:     -ms-linear-gradient(top, #2f2f2f, #4f4f4f);
background-image:      -o-linear-gradient(top, #2f2f2f, #4f4f4f);
background-image:      linear-gradient(top, #2f2f2f, #4f4f4f);
width: 100%;
}
.top-nav {
border-bottom: 2px solid #111;
height: 30px;
list-style-type: none;
margin: 0;
padding-left: 0;
width: 100%;
}
.top-nav li {
background: #333;
background-image: -webkit-gradient(linear, 0% 0%, 0% 100%, from(#2f2f2f), to(#4f4f4f));
background-image: -webkit-linear-gradient(top, #2f2f2f, #4f4f4f); 
background-image:    -moz-linear-gradient(top, #2f2f2f, #4f4f4f);
background-image:     -ms-linear-gradient(top, #2f2f2f, #4f4f4f);
background-image:      -o-linear-gradient(top, #2f2f2f, #4f4f4f);
background-image:      linear-gradient(top, #2f2f2f, #4f4f4f);
border-bottom: 2px solid #111;
border-right: 1px solid #555;
float: left;
font-size: 14px;
height: 20px;
padding-top: 10px;
position: relative;
text-align: center;
width: 150px;
}
.top-nav li ul.drop-down {
position: absolute;
top: 32px;
left: 0;
visibility: hidden;
display: none;
opacity: 0;
-webkit-transition: opacity 0.3s;
-moz-transition: opacity 0.3s;
-o-transition: opacity 0.3s;
transition: opacity 0.3s;
}
.top-nav li:hover ul.drop-down {
visibility: visible;
opacity: 1;
display: block;
}
.top-nav li .drop-down li {
height: 20px;
-webkit-transition: height 0.3s ease;
-moz-transition: height 0.3s ease;
-o-transition: height 0.3s ease;
transition: height 0.3s ease;
}
.top-nav li .drop-down li:hover {
height: 30px;
}
.top-nav li a {
color: #aaa;
padding-top: 5px;
position: absolute;
top: 0;
left: 0;
width: 150px;
height: 25px;
text-decoration: none;
}

source d'informationauteur Vladimir