So apparently, this is not possible? What's the reason?

.xxx:hover, .yyy:hover, .zzz:hover {
filter:alpha(opacity=50);
zoom: 1;
opacity:0.5;
}

The css works fine, that is not my issue so no comment needed there, but as soon as you stack several hover classes for 1 definition, it breaks down. Whereas you can stack other styles on a single definition no problem.

What am I missing in this case?