CSS3 :hover animation has z-index bug - debugging

Question

Ok, so I have an empty <span> that is nested in side my <li>'s of an unordered list. The span holds a background image that is supposed to appear on :hover. The problem is that while the animation is transitioning, the z-index is wrong, the <span> stacks itself in front of the <a> element which precedes it in the DOM. As soon as the animation completes, however, the stacking order corrects itself. The result is a visual sudden "snap" of the effect and also the link becomes unclickable for the duration of the CSS3 transition.
Can anyone break down what is happening at the DOM level? How can I fix this?
You can see a working example that demonstrates the issue here: http://jsfiddle.net/qZkfw/1/
My HTML
<div id="nav">
<ul id="nav-main">
<li>Home<span></span></li>
<li>About<span></span></li>
<li>Get Fit<span></span>
<ul class="nav-secondary">
<li>Exercise Library</li>
<li>Find An Instructor</li>
<li>Fitness Tools</li>
</ul>
</li>
<li>Find An Instructor<span></span></li>
<li>Get Certified<span></span></li>
</ul>
</div>
my CSS
#nav-main li {
margin:0;
padding:5px;
position: relative;
display: block;
float: left;
margin-left: 10px;
}
#nav-main li a {
color: #97dd6e;
font-size: 1.1em;
text-decoration: none;
text-align: center;
margin-right: -7px;
line-height: 19px;
z-index: 99;
}
#nav-main li span {
height: 28px;
background: transparent url('/images/application/bg_nav_active_repeat.png') repeat-x top center;
display: block;
margin: -22px -5px 0 2px;
padding: 3px 0 0 0;
z-index: 98;
opacity: 0;
visibility: hidden;
-webkit-transition: all 0.2s ease-in-out;
-moz-transition: all 0.2s ease-in-out;
-o-transition: all 0.2s ease-in-out;
transition: all 0.2s ease-in-out;
}
#nav-main li:hover span {
opacity: 1;
visibility: visible;
-webkit-transition: all 0.4s ease-in-out;
-moz-transition: all 0.4s ease-in-out;
-o-transition: all 0.4s ease-in-out;
transition: all 0.4s ease-in-out;
}
#nav-main li span:before, #nav-main li span:after {
content: '';
height: 28px;
width: 7px;
display: block;
background: transparent url('/images/application/bg_nav_active_before.png') no-repeat left top;
position: absolute;
top: 2px;
left: 0;
z-index: 999;
}
#nav-main li span:after {
background: transparent url('/images/application/bg_nav_active_after.png') no-repeat right top;
left: 100%;
}

Answer 1

Add position:relative; z-index: -1; to #nav-main li span.
Updated jsfiddle.
Edit:
I figured it out.
Static elements do not obey z-index, so you need to add position:relative; to #nav-main li a.
Updated jsfiddle.