Firefox对hover、active的兼容问题
ff4以上写法：
.x-bg {background-color: #fff;}.x-bg:hover {background-color: #ff000;}.x-bg:active {background-color: #000;}.x-bg {background-color: #fff;}div.x-bg:hover {background-color: #ff000;}div.x-bg:active {background-color: #000;}