When setting opacity to an element in IE8+, according to http://www.quirksmode.org/css/opacity.html I can use

.opacity {
    filter: progid:DXImageTransform.Microsoft.Alpha(Opacity=50);
    opacity: 0.5;
}

note that the opacity in IE8 filter is in upper case

but jQuery 1.* currently only support getting the result setted in lower case opacity, like filter: progid:DXImageTransform.Microsoft.Alpha(opacity=50);

see demo http://jsfiddle.net/c9d5ctfa/ in IE8

Also in MSDN, both upper case and lower case are used in the examples.

My PR: #1704