css 完全居中

css完全居中一

width: 50px;
height: 50px;
background-color: #000;
position: absolute;
margin:auto;
left:0; top:0; bottom:0; right:0;

css完全居中二

width: 50px;
height: 50px;
background-color: #000; 
position: absolute;
left: 50%;
margin-left: -25px;
top:50%;
margin-top:-25px;