http://blog.csdn.net/cooleddie1/article/details/8622972
background:rgba(0,0,0,0.5);/*前三个参数为rgb颜色，第四个参数为透明度*/
filter:progid:DXImageTransform.Microsoft.gradient(startcolorstr=#7F000000,endcolorstr=#7F000000);/*#7F000000的前两位为16进制透明度，后六位为16进制颜色*/