grayscale img with css crossbrowser

thumbnail.css

a.a-thumb {
	border: 1px solid black;
	position: relative;}
a.a-thumb img {
	width: 60px;
	height: 60px;
	border: 0;}
a.a-thumb span {
	background-color: #000000;
	position: absolute;
	top: 0;
	left: 0;
	width: 60px;
	height: 60px;
	z-index: 100;
	filter: alpha(opacity=20);
	-moz-opacity: 0.2;

thumbnail.html

<a class="a-thumb"><img src="/img/proxy-thumb.gif" /><span> </span></a>