Please have a look at Microsoft's Map feature: http://local.live.com

If you drag the map around in IE6, you can see that the top bar is actually translucent.

This is the CSS they use for the trick:

#wl_ve_header
{
...
background: rgb(255,255,255);
...
filter: alpha(opacity=90);
opacity: 0.9;
...
}

Does anyone recommend or not recommend using these rules? Any comments? Are these rules valid CSS?

If these are safe, I will use them in my IE-only conditional-comment-filtered stylesheets. Would be interested in other people's thoughts.

I think only IE supports that rubbish.

the filter rule is IE only (most of the time filter rules like that don't validate), the opacity rule is in some version of the CSS spec (so it should validate, but i don't think there's any browsers that support it right now), mozilla supports -moz-opacity (which doesn't validate).

filter:progid:DXImageTransform.Microsoft.Alpha(opacity=50);

yeah, that's the one that doesn't validate. too many :s in it. but it does work.