There is a design where a button need to be made "more invisible", either by creating another image with brightness +192 or +212, or we can use CSS's opacity
to simulate the same effect.
It seems that with the special situation of IE (6, 7, and 8+), the following:
can make the opacity happen in almost all browsers, including IE 6? So really, there is no need to add one more image, but just using CSS is good enough?
According to this example you can do more. Especially for IE8:
/* IE 8 */ -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";
More info why MS did it this way in article Opacity rettuns to IE8