a:active not working

I've made a menu bar that works mostly through CSS. I see a problem in
Netscape 7 for Mac. In my latest attempt, the "button" doesn't change
color when you click on it. 

In an earlier attempt,
(http://iplab.com/webtest/csstest7.html),
the links change color when you hover over them and when you activate
them by clicking (click and hold to see this). This page used text
instead of images.

In my current attempt,
(http://iplab.com/webtest/csstest8.html),
which uses images for the menu items, the image backgrounds change
color when you hover over them. In Netscape 7/Mac, the backgrounds do
not change color when you click and hold. In IE 5.1/Mac, however, the
backgrounds DO change color when you click and hold. 

Since I know this functionality is supported in N7, I think I must be
missing a mistake. The w3c CSS validator didn't turn up a mistake for
me, so I turn to you.

Any commentary on color choices or the like is also welcome.

Thanks!

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