1-div上下左右居中
方法1:定宽高:
position:absolute;
top:50%;
left:50%;
margin-top:(width/2);
top:50%;
left:50%;
margin-top:(width/2);
margin-left:(width/2);
方法2:
position:absolute;
top:50%;
left: 50%;
-webkit-transform: translate(-50%, -50%);
transform: translate(-50%, -50%);
方法3:
position:absolute;
left:0;
top:0;
right:0;
bottom:0;
margin:auto;
方法4:
display: flex;
align-items: center;
justify-content: center;