In most cases we can achieve this effect purely with CSS, but since we have to attend to our red headed step child aka IE6, we will use a few lines of jQuery to cover all grounds.

Wireframe – HTML

Nest a set of links wrapped within the <span> tag, this is how the sub navigation will be positioned.

<ul id="topnav">
    <li><a href="#">Link</a></li>
    <li>
        <a href="#">Link</a>
        <!--Subnav Starts Here-->
        <span>
            <a href="#">Subnav Link</a> |
            <a href="#">Subnav Link</a> |
            <a href="#">Subnav Link</a>
        </span>
        <!--Subnav Ends Here-->
    </li>
    <li><a href="#">Link</a></li>
</ul>

Styling – CSS

Unlike a regular drop down menu where the subnav appears directly underneath the hovered/clicked list item, in this case all the subnav sets will start at the same location (left aligned – underneath the navigation).

ul#topnav {
	margin: 0; padding: 0;
	float: left;
	width: 970px;
	list-style: none;
	position: relative; /*--Set relative positioning on the unordered list itself - not on the list item--*/
	font-size: 1.2em;
	background: url(topnav_stretch.gif) repeat-x;
}
ul#topnav li {
	float: left;
	margin: 0; padding: 0;
	border-right: 1px solid #555; /*--Divider for each parent level links--*/
}
ul#topnav li a {
	padding: 10px 15px;
	display: block;
	color: #f0f0f0;
	text-decoration: none;
}
ul#topnav li:hover { background: #1376c9 url(topnav_active.gif) repeat-x; }
/*--Notice the hover color is on the list item itself, not on the link. This is so it can stay highlighted even when hovering over the subnav--*/

Now set the absolute positioning on the <span> tag 35px from the top. I added some rounded corners on the bottom for style (this will not work in IE).

ul#topnav li span {
	float: left;
	padding: 15px 0;
	position: absolute;
	left: 0; top:35px;
	display: none; /*--Hide by default--*/
	width: 970px;
	background: #1376c9;
	color: #fff;
	/*--Bottom right rounded corner--*/
	-moz-border-radius-bottomright: 5px;
	-khtml-border-radius-bottomright: 5px;
	-webkit-border-bottom-right-radius: 5px;
	/*--Bottom left rounded corner--*/
	-moz-border-radius-bottomleft: 5px;
	-khtml-border-radius-bottomleft: 5px;
	-webkit-border-bottom-left-radius: 5px;
}
ul#topnav li:hover span { display: block; } /*--Show subnav on hover--*/
ul#topnav li span a { display: inline; } /*--Since we declared a link style on the parent list link, we will correct it back to its original state--*/
ul#topnav li span a:hover {text-decoration: underline;}

For those who are not very familiar with how positioning works, check out the following tutorials:

• w3schools – CSS Positioning
• Absolute, Relative, Fixed Positioning: How Do They Differ?
• Stopping the CSS positioning panic

IE6 Fix – jQuery

Since IE6 does not understand li:hover (basically it only understands a hover event on a <a> tag), use jQuery to go around the issue.

For those who are not familiar with jQuery, do check out their site first and get an overview of how it works. I’ve shared a few tricks that I have picked up along the way, you can check those out as well.

Initial Step – Call the jQuery file
You can choose to download the file from the jQuery site, or you can use this one hosted on Google.

<script type="text/javascript"
src="http://ajax.googleapis.com/ajax/libs/jquery/1.3/jquery.min.js"></script>

The following script contains comments explaining which jQuery actions are being performed.

<script type="text/javascript">
$(document).ready(function() {

	$("ul#topnav li").hover(function() { //Hover over event on list item
		$(this).css({ 'background' : '#1376c9 url(topnav_active.gif) repeat-x'}); //Add background color and image on hovered list item
		$(this).find("span").show(); //Show the subnav
	} , function() { //on hover out...
		$(this).css({ 'background' : 'none'}); //Ditch the background
		$(this).find("span").hide(); //Hide the subnav
	});

});
</script>