this.offsetParent返回包含这个元素的元素，offsetTop和offsetLeft就是相对于offsetParent定义的,看看下面的代码你就明白了:
<HTML>
<HEAD>
<TITLE>Elements: Positions</TITLE>
<SCRIPT LANGUAGE="JScript">
function showPosition()
{
var oElement = document.all.oCell;
alert("The TD element is at (" + oElement.offsetLeft +
"," + oElement.offsetTop + ")\n" + "The offset parent is "
+ oElement.offsetParent.tagName );
}
</SCRIPT>
</HEAD>
<BODY onload="showPosition()">
<P>This document contains a right-aligned table.
<TABLE BORDER=3 ALIGN=right>
<TR>
<TD ID=oCell>This is a small table.</TD>
</TR>
</TABLE>
</BODY>
</HTML>
你可以把table 的border设得大点或小点试试
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
