元素的margin为负值时有一些特定的规则:

• A negative margin to the left or top will pull the element in that direction,
  overlapping any elements next to it.
• A negative right or bottom margin will pull in any adjacent elements so that they
  overlap the element with the negative margin.
• On a floated element, a negative margin opposite the float direction will decrease
  the float area, causing adjacent elements to overlap the floated element. A negative
  margin in the direction of the float will pull the floated element in that direction.
• Finally, the behavior of negative margins to the sides is slightly moderated when
  used on a nonfloating element without a defined width. In that case, negative
  margins to the left and right sides both pull the element in that direction. This
  expands the element, potentially overlapping any adjacent elements.