#wrap{width:970px; margin:0px;auto}
#mainbody{position:relative; margin-top:15px;}
#left{float:left;width:110px; }
#mid{margin-left:123px;width:650px;border:1px solid #999;}
#right{position:absolute;top:0px;left:790px;border:1px solid #999;}

123px  因为父元素有relative，子元素有absolute，margin-left相对于父元素左侧

#wrap{width:970px; top:0;}

#mainbody{position:relative; margin-top:15px;}

#left{width:110px; }

#mid{margin-left:123px;width:650px;border:1px solid #999;top:0;position:absolute;}

#right{position:absolute;margin-left:790px;border:1px solid #999;top:0;}