offsetParent --Retrieves a reference to the container object that defines the offsetTop and offsetLeft properties of the object.
有一些具体的细节:
1.for ie5.0 and above,the offsetParent property return the table object for the td object in ie4.0 it returns the tr object
2.大多数情况下offsetParent得到的是body.div ,span具有容器这种特性
3.offsetParent与position有很大的关系