属性名称不存在时，最好先用hasAttribute检测下：
Essentially all web browsers (Firefox, Internet Explorer, recent versions of Opera, Safari, Konqueror, and iCab, as a non-exhaustive list) return null when the specified attribute does not exist on the specified element. The DOM specification says that the correct return value in this case is actually the empty string, and some DOM implementations implement this behavior. The implementation of getAttribute in XUL (Gecko) actually follows the specification and returns an empty string. Consequently, you should use hasAttribute to check for an attribute's existence prior to calling getAttribute() if it is possible that the requested attribute does not exist on the specified element.
链接：https://developer.mozilla.org/en/DOM/element.getAttribute#Notes