题目：
.a{
width: 200px;
height: 200px;
background-color: #ccc;
}
<body>
<div class="a">
<div class="b">1111</div>
</div>
</body>
css3的transform
.a{
position: relative;
}
.b{
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
}
2、第二种 display的table-cell
.a{
display: table;
}
.b{
display: table-cell;
text-align: center;
vertical-align: middle;
}
3、第三种 flex盒子布局
.a{
display: flex;
justify-content: center;
align-items: center;
}