A principal block box becomes the containing block for its descendant boxes, provided that they are statically or relatively positioned. For floating and absolutely positioned boxes other rules apply.

• 假如一个关键块级盒子被静止或相对定位了；它就会成为它里面后代盒子的容器.；但浮动和绝对定位的盒子就会遵循另外的规则了