下面的文案注释来自：http://www.w3.org/TR/2006/WD-CSS21-20061106/visuren.html#propdef-position
relative The box's position is calculated according to the normal flow (this is called the position in normal flow). Then the box is offset relative to its normal position. When a box B is relatively positioned, the position of the following box is calculated as though B were not offset. The effect of 'position:relative' on table-row-group, table-header-group, table-footer-group, table-row, table-column-group, table-column, table-cell, and table-caption elements is undefined.
table定义position:relative属性无效。 可以给td加div，给div再设置
