为啥margin-left:auto可以使div元素靠右

The following constraints must hold among the used values of the other properties:
'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' +'border-right-width' + 'margin-right' = width of containing block

总共加起来需要等于块的宽度,左边已经被占了,那么auto就自动占据剩下的了