效果预览
以下为源码
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<style> *{ padding:0; margin:0 } .menu>li{ float:left; width:80px; background:#0ff; list-style:none; text-align: center; } .menu>li:hover .hiddenlist{ height:100px; } .menu>li:hover{ background:#707; } .hiddenlist{ position:fixed; height:0; overflow:hidden; background:#999; transition:height 0.5s ; } .hiddenlist ul{ position:relative; top:50%; left:50%; transform:translate(-50%,-50%); width:80px; } .hiddenlist ul li{ text-align:center; } .hiddenlist li:hover { background:#7fff; } @keyframes move{ 0% {height: 0 } 100% {height: 120px} } </style>
<body>
<div class="nav">
<ul class="menu">
<li>一
<div class="hiddenlist">
<ul>
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
</ul>
</div>
</li>
<li>二
<div class="hiddenlist">
<ul>
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
</ul>
</div>
</li>
<li>三
<div class="hiddenlist">
<ul>
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
</ul>
</div>
</li>
</ul>
</div>
<div style="clear:both;">因为hidden的position属性为fixed，所以不会影响布局</div>
</body>
</html>