CSS bottom border hover "jitter"

Question

I have a navigation bar which I would like to give an orange bottom border when you hover over the navigation buttons. Only problem is that whenever you hover, the border makes the content/navigation buttons "jitter" which they aren't supposed to. Also I already have a black bottom border on the navigation bar at all times so it wont work to change that.

HTML:

<div id="navBarTop">
        <ul>
                <li><a href="index.html">Home</a></li>
                <li><a href="contact.html">Contact</a></li>
        </ul>
</div>

CSS:

#navBarTop {
    padding: 0;
    background-image: url('navBarBackground1.png');
    border-bottom: 1px solid #4c4c4c;
    position: absolute;
    bottom: 0;
    text-align: center;
    left: 0;
    right: 0;
}

#navBarTop ul {
    list-style: none;
    width: 800px;
    margin: 0 auto;
    padding: 0; 
    display: inline-block;
}

#navBarTop li {
    display: inline-block;
}

#navBarTop li a {
    display: block;
    padding: 10px 25px;
    text-decoration: none;
    font-family: "Arial";
    color: #ffffff;
}

#navBarTop li a:hover {
    border-bottom: 2px solid #FF8000;
}

Answer 1

The jittering seems to be caused by adding the extra 2px border at the bottom when you hover. That causes the text to rise up a bit. To fix this, make sure there's always a 2px border by changing your #navBarTop li a to this:

#navBarTop li a {
    display: block;
    padding: 10px 25px;
    text-decoration: none;
    font-family: "Arial";
    color: #00ffff;
    border-bottom: 2px solid transparent;  // <--- add this line
}

That should stabilize things for you.