Steps to Reproduce:

Assign a background gradient to an element in your external CSS, like so:

filter: progid:DXImageTransform.Microsoft.gradient(startColorStr='#ffff0000', EndColorStr='#ffffffff');

On the same element, set a moderate opacity:

$(elem).css('opacity', '0.5');

Your gradient will disappear.

Steps to Fix:

1. Use

elem.currentStyle['filter']

2. If the Alpha filter isn't already applied, add to the filter list (space- or comma-delimited)

3. If it does exist previously, modify via

elem.filters["DXImageTransform.Microsoft.Alpha"].opacity = Number_between_0_and_100