汉堡按钮的实现
汉堡按钮的实现思想是：button内包含一个span，设置span为绝对定位，并设置背景颜色，使之成为汉堡按钮的夹心，
同理，再用span:before、span:after，创造一上一下两片面包，具体实现如下：
<body>
<button class="hamburger">
<span>menu</span>
</button>
</body>
.hamburger{
display: block;
position: relative;
font-size: 0;
width: 48px;
height: 48px;
border: none;
cursor: pointer;
}
.hamburger:focus{
outline: none;
}
.hamburger span{
display: block;
position: absolute;
height: 4px;
top: 22px;
left: 8px;
right: 8px;
background-color: #fff;
}
.hamburger span:before,
.hamburger span:after{
content:"";
display: block;
position: absolute;
width: 100%;
height: 4px;
background: #fff;
}
.hamburger span:before{
top: -10px;
}
.hamburger span:after{
bottom: -10px;
}
