<style>

.dropdown {

position: relative;

display: inline-block;

}

.dropdown-content {

display:none;

position:absolute;

background-color:#f9f9f9;

min-width:80px;

box-shadow:0px 8px 16px 0px rgba(0,0,0,0.2);

padding:12px 16px;

z-index:1;

}

.dropdown:hover .dropdown-content {

display:block;/*块级元素*/

}

</style>

<div class="dropdown">

<span class="bg_white"></span>

<div class="dropdown-content">

<p><a href="http://www.baidu.com">登录</a></p>

<p><a hre="http://www.baidu.com">注册</a></p>

<p><a href="http://www.baidu.com">退出</a>

</p>

</div>

</div>