jQuery XPath 介绍
jQuery XPath 是一个 jQuery 插件，实现了全功能的 XPath 2.0 查询语言。
示例代码：
$(document).xpath("*"); // Returns {Element} html (direct child of context item - document)
$(document).xpath("//head << //body"); // Returns {Boolean} true (head is preceding body)
$(document).xpath("//*[parent::html][last()]") // Returns {Element} body (last child of html)
$(document.body).xpath("count(ancestor::node())"); // Returns {Number} 2 (2 ancestor nodes)
$(document.body).xpath("preceding-sibling::element()"); // Returns {Element} head (prev sibling)
$(document.documentElement).xpath("body | head"); // Returns {Element} head and body (ordered)
$(document.documentElement).xpath("body, head"); // Returns {Element} body and head (not ordered)