为什么调用startMove函数时，对象只能是li1，而不能是this

 var li1=document.getElementById('li1');

            li1.onmouseover= function () {
                startMove(li1,'width',400,function(){
                    startMove(li1,'height',200,function(){
                        startMove(li1,'opacity',100);
                    });
                });
            };
            li1.onmouseout= function () {
                startMove(li1,'opacity',30,function(){
                    startMove(li1,'height',100,function(){
                        startMove(li1,'width',200);
                    });
                });
            };