I'm using jQuery's addClass to add a class to an tab so when clicked it'll change the class so it's obvious the tab is highlighted.

My HTML

<div id="mainNav">
            <ul>
                <li id="LinktheBand" onclick="nav('theBand')">The Band</li>
                <li id="linkTheMusic" onclick="nav('theMusic')">The Music</li>

My CSS

#mainNav li {
    float:left;
    margin-right:5px;
    color:white;
    font-family:Tahoma;
    font-weight:bold;
    padding: 10px 8px 0px 8px;
    background:url('../images/transparent-65.png');
    height:40px;
    height: 25px !important;
    border:1px solid #000;
}

#mainNav li:hover {
    float:left;
    margin-right:5px;
    color:white;
    font-family:Tahoma;
    font-weight:bold;
    padding: 10px 8px 0px 8px;
    background: #660000;
    height:40px;
    height: 25px !important;
    border:1px solid #660000;
}

.mainNavSelected {
    float:left;
    margin-right:5px;
    color:white;
    font-family:Tahoma;
    font-weight:bold;
    padding: 10px 8px 0px 8px;
    background: #660000;
    height:40px;
    height: 25px !important;
    border:1px solid #660000;
}

My Javascript

function nav(a) {
$('#'+a).show();
$('#Link'+a).addClass('mainNavSelected');
}

This works properly, I check firebug and can see the class="mainNavSelected" is added, but the list element doesn't take any properties of the new class. Firebug lists all the class items for mainNavSelected, but has them all crossed out. What am i missing to replace the class of this element?