1.flex(不用宽高)
父元素 display: flex; justify-content: center; align-items: center;
3.translate(不用宽高)
父 position: relative;
子 position: absolute; top: 50%; left: 50%; transform: translate(-50%, -50%);
子元素 display: inline / inline-block;
4.absolute 负边距
父 position: relative;
子 position: absolute; left: 50%; top: 50%; margin-left: -子元素宽/2; margin-top: -子元素高/2;
5.absolute 0
父 position: relative;
子 position: absolute; top: 0; bottom: 0; left: 0; right: 0; margin: auto; 50%; height: 50%;
5.table-cell(不用宽高)
父元素 display: table-cell; vertical-align: center; text-align: center;
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