针对div
display: flex;
align-items: center;
justify-content: center;
绝对定位+transform
position:absolute;
top: 50%;
left: 50%;
transform:translate(-50%,-50%);
绝对定位+margin：auto
position:absolute;
left:0;
top:0;
right:0;
bottom:0;
margin:auto
绝对定位+margin反向偏移
position：absolute;
top:50%;
left:50%;
margin-left:-(width+padding)/2+'px';
margin-top:-(height+padding)/2+'px';
针对单行的文本
line-height同height
text-align: center