Демо:
http://jsfiddle.net/34uqv/<html>
<head>
<style type="text/css">
ul.parent li{
display: inline;
float:left;
width:90px;
height:20px;
border:1px solid gray;
z-index:1;
text-align: center;
cursor:pointer;
}
ul.parent li:hover{
background-color: black;
color:white;
}
ul.child1{
display:none;
width: 400px;
}
ul.child1 li{
position:relative;
left:-41px;
display:inline;
float: left;
z-index:2;
text-align: center;
background-color: black;
color:white;
}
ul.child1 li:hover{
background-color: gray;
color:black;
}
ul.child2{
display:none;
width: 400px;
}
ul.child2 li{
position:relative;
left:-91px;
display:inline;
float: left;
z-index:2;
text-align: center;
background-color: black;
color:white;
}
ul.child2 li:hover{
background-color: gray;
color:black;
}
ul.child3{
display:none;
width: 400px;
}
ul.child3 li{
position:relative;
left:-225px;
display:inline;
float: left;
z-index:2;
text-align: center;
background-color: black;
color:white;
}
ul.child3 li:hover{
background-color: gray;
color:black;
}
</style>
<script type="text/javascript">
function show(parent,child_id,action){
var parent = this;
var child = document.getElementById(child_id);
if(action==1)
child.style.display = 'block';
else
child.style.display = 'none';
}
</script>
</head>
<body>
<ul class="parent">
<li onmouseover="show(this,'child1',1)" onmouseout="show(this,'child1',0)"> Первый
<ul id="child1" class="child1">
<li>Первый 1</li>
<li>Первый 2</li>
<li>Первый 3</li>
</ul>
</li>
<li onmouseover="show(this,'child2',1)" onmouseout="show(this,'child2',0)">Второй
<ul id="child2" class="child2">
<li>Второй 1</li>
<li>Второй 2</li>
</ul>
</li>
<li onmouseover="show(this,'child3',1)" onmouseout="show(this,'child3',0)">Третий
<ul id="child3" class="child3">
<li>Третий 1</li>
<li>Третий 2</li>
<li>Третий 3</li>
<li>Третий 4</li>
</ul>
</li>
</ul>
</body>
</html>
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