<div class="father">
<div class="sun"></div>
</div>
第一种
.father{
width: 500px;
height: 500px;
background-color: #cccccc;
position: relative;
}
.sun{
width: 100px;
height: 100px;
background-color: #ffffff;
position: absolute;
top: 0;
right: 0;
bottom: 0;
left: 0;
margin: auto;
}
第二种
.father{
width: 500px;
height: 500px;
background-color: #cccccc;
position: relative;
}
.sun{
width: 100px;
height: 100px;
background-color: #ffffff;
position: absolute;
margin: auto;
top: 50%;
left: 50%;
transform: translate(-50%,-50%);
}
第三种（弹性盒子）
.father{
width: 500px;
height: 500px;
background-color: #cccccc;
display: flex;
justify-content: center;
align-items: center;
}
.sun{
width: 100px;
height: 100px;
background-color: #ffffff;
}
