In addition to @arminb's Answer (I can't just comment on it) display:none will work in all (vaguely modern) browsers, opacity:0 has flaky support in older browsers (doesn't work in IE6, 7 or 8)
If you do need Opacity then something like this should work in all:
.opaque {
-ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=0)";
filter: alpha(opacity=0);
opacity: 0;
}
But that's messy. display:none is the cleanest solution which will work. visibility:hidden will also work in more browsers than opacity and achieves the same purpose (i.e. makes it invisible, but leaves it there)
