[CSS] IE8 이하 opacity 적용 방법

적는 순서도 중요!!

-ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";   // IE8

filter: alpha(opacity=50);     // IE7

opacity:0.5;                   // all browsers

[참고] http://www.quirksmode.org/css/opacity.html