a:active not working in navigation

I'm new to this so it's probably a really stupid mistake I've made, but I'd be really grateful if nyone can help! I'm making a website for which I'm using a style sheet. At the top of the page there is a navigation bar which has a black background and is grey when hovered over but should also be grey when a link is active. As far as I can tell, it should be working, but it isn't. Any ideas?
Here's the CSS

/* Default CSS */
/* Generated by the CoffeeCup HTML Editor - <a href="http://www.coffeecup.com" rel="nofollow">www.coffeecup.com</a> */
background-image:url('Bryn Background.jpg');

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