Opposite horizontal margin constraint. A left float box that has another left float box to its left may not have its right marginedge to the right of the containing block's right padding edge.
Left - Increase the generated box's top margin so that the top border edge is below the bottom marginedge of any left float boxes generated from previous source elements.
left -增加所生成框的上边框，以便上border edge处于由之前的源元素所生成的任一左浮动框的底margin edge之下。