https://github.com/mathiasbynens/jquery-placeholder
解决ie不支持HTML5 placeholder的问题
Requires jQuery 1.6+. For an older version of this plugin that works under jQuery 1.4.2+, see v1.8.7.Works in all A-grade browsers, including IE6.
来源：oschina
链接：https://my.oschina.net/u/124822/blog/318377