为元素为什么display：table呢？ 不能使block
涉及到margin折叠。
W3C：http://www.w3.org/TR/CSS2/tables.html#anonymous-boxes
Any table element will automatically generate necessary anonymous table objects around itself, consisting of at least three nested objects corresponding to a 'table'/'inline-table' element, a 'table-row' element, and a 'table-cell' element.
所以display:table 会产生匿名的table-cell元素，进而产生BFC从而避免margin折叠。
It does not need to be display: table. The difference, however, is worth to be mentioned: If you use display: block instead of display: table, your margins will collapse as usual. If you use display: table, however, you will lose margin collapse between siblings and the first/last children of the element where you've applied the clearfix on, which might or might not be a desired feature.