opacity用法:
/* older safari/Chrome browsers */
-webkit-opacity: 0.5;
/* Netscape and Older than Firefox 0.9 */
-moz-opacity: 0.5;
/* Safari 1.x (pre WebKit!) 老式khtml内核的Safari浏览器*/
-khtml-opacity: 0.5;
/* IE9 + etc...modern browsers */
opacity: .5;
/* IE 4-9 */
filter:Alpha(opacity=50)