CSS3 animation transition: opacity not working - html

Question

i'm having some trouble with animating a dropdown menu using css3. I need it to work with css3 and not jQuery or javascript. I've added all the rules neccessary but still the effect doesn't happen. Can anyone please help? Here is a fiddle of my code. Thank you.
http://jsfiddle.net/Zmr7u/6/
html code:
<header class="main-header">
<nav class="main-nav">
<ul class="top-nav">
<li>
home
<ul class="drop-down">
<li>1</li>
<li>2</li>
<li>3</li>
</ul>
</li>
<li>
about
<ul class="drop-down">
<li>1</li>
<li>2</li>
<li>3</li>
</ul>
</li>
<li>
products
<ul class="drop-down">
<li>1</li>
<li>2</li>
<li>3</li>
</ul>
</li>
<li>
contacts
<ul class="drop-down">
<li>1</li>
<li>2</li>
<li>3</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;
}

Answer 1

You can just use visibility without using display:
.top-nav li ul.drop-down {
list-style-type: none;
position: absolute;
top: 32px;
left: -40px;
visibility: hidden;
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;
}
Demo.

Answer 2

You can use css animation when you want to increase opacity and make the element visible, like --
span {
display: none;
opacity: 0;
}
To make span visible
span {
display: block;
animation: visible 2s;
}
#keyframes visible {
from {
opacity: 0;
}
to {
opacity: 1;
}
}
And if you're getting a bug when make animation less than 1s then use animation: visible 0.5s forwards;