先看W3C文档
In a block formatting context, boxes are laid out one after the other, vertically, beginning at the top of a containing block. The vertical distance between two sibling boxes is determined by the'margin' properties. Vertical margins between adjacent block-level boxes in a block formatting context collapse.
In a block formatting context, each box's left outer edge touches the left edge of the containing block (for right-to-left formatting, right edges touch). This is true even in the presence of floats (although a box's line boxes may shrink due to the floats), unless the box establishes a new block formatting context (in which case the box itself may become narrower due to the floats).
Block Formatting Context，中文直译为块级格式上下文。BFC就是一种布局方式，在这种布局方式下，盒子们自所在的containing block顶部一个接一个垂直排列，水平方向上撑满整个宽度（除非内部盒子自己建立了新的BFC）。两个相邻的BFC之间的距离由margin决定。在同一个BFC内部，两个垂直方向相邻的块级元素的margin会发生“塌陷”。
