火狐官网上找到的一组函数，相当于treeWalker，有了它可以方便地在IE实现Traversal API 2的所有功能（nextElementSibling，previousElementSibling，firstElementChild，lastElementChild，children）These functions let you find the next sibling, previous sibling, first child, and last child of a given node (element). What makes them unique is that they safely ignore whitespace nodes so you get the real node you're looking for each time.
function is_all_ws(nod) { return !(/[^\t\n\r ]/.test(nod.data)); }
function is_ignorable(nod) { return (nod.nodeType == 8) || ((nod.nodeType == 3) && is_all_ws(nod)); }
function node_before(sib) {
while ((sib = sib.previousSibling)) {
if (!is_ignorable(sib)) return sib;
}
return null;
}
function node_after(sib) {
while ((sib = sib.nextSibling)) {
if (!is_ignorable(sib)) return sib;
}
return null;
}
function first_child(par) {
var res = par.firstChild;
while(res) {
if(!is_ignorable(res)) return res;
res = res.nextSibling;
}
return null;
}
function last_child(par) {
var res = par.lastChild;
while(res) {
if(!is_ignorable(res)) return res;
res = res.previousSibling;
}
return null;
}