#box {
	opacity: 0.5;
	filter:alpha(opacity=50);
	-khtml-opacity: 0.5;
	-moz-opacity:0.5;
}

/* opacity: 0.5; = current CSS standard for opacity*/
/* filter:alpha(opacity=50); = IE*/
/* -khtml-opacity: 0.5; = older versions of Safari*/
/* -moz-opacity:0.5; = older mozilla browsers such as Netscape Navigator*/