方式一:
.box1{
position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
200px;
height: 200px;
background-color: yellowgreen;
margin: auto;
}
方式二:
.box2{
position: absolute;
top: 50%;
left: 50%;
margin-left: -100px;
margin-top: -100px;
200px;
height: 200px;
background-color: yellowgreen;
}
方式三:
.box3{
200px;
height: 200px;
background-color: yellowgreen;
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
}