CSS opacity

/* This works in IE */
filter: alpha(opacity=50);

/* Older than Firefox 0.9 */
-moz-opacity:0.5;

/* Safari 1.x (pre WebKit!) */
-khtml-opacity: 0.5;

/* Modern!
/* Firefox 0.9+, Safari 2?, Chrome any?
/* Opera 9+, IE 9+ */
opacity: 0.5;

Copy:

filter: alpha(opacity=50);
-moz-opacity:0.5;
-khtml-opacity: 0.5;
opacity: 0.5;