To cover all browsers you need to a have more than one setting for transparency in the CSS. Its getting a little better if you ignore very old browsers but you still need two settings to cover IE V 7, 8, 9, plus Firefox, Chrome, Safari and Opera...
.transparent {
/* Theoritically required for IE 9 and is more valid but filter also works*/
-ms-filter: "progrid:DXImageTransform.Microsoft.Alpha(Opacity=50)";
/* For IE 5, 6, 7, but aloso works in IE8 and 9*/
filter: alpha(opacity=50);
/* For all modern browsers including IE 9*/
opacity: 0.5;
}
Since iWeb is not being developed any more, I doubt there will be an update to cover the idiosyncrasies of IE 9! Yet another reason to find another way to create websites.