DOM兼容

-firstChild  firstElementChild

var oFirst = oUl.firstChild || oUl.firstElementChild;

-lastChild   lastElementChild

var oLast = oUl.lastChild || oUl.lastElementChild;

-nextSibling  nextElementSibling;

var oNext = oUl.nextSibling || oUl.nextElementSibling;

-previousSibling   previousElementSibling

var oPre = oUl.previousSibling || oUl.previousElementSibling;