/*Making an element transparent can be accomplished using only CSS, 
but requires about five lines of code. With jQuery, we can achieve 
transparency with just one line: */
$('selector').css('opacity', 0.5);

/* below is the CSS way, and it's sloppy
selector {
	filter: alpha(opacity=50);	/* internet explorer * /
	-khtml-opacity: 0.5;		/* khtml, old safari * /
	-moz-opacity: 0.5;			/* mozilla, netscape * /
	opacity: 0.5;				/* fx, safari, opera * /
}
*/

Cross Browser Opacity

jquery

JavaScript