HTML - CSS: cross-browser semi-transparency

I've to make the background of my fading-in menu items semi-transparent.

It has to be grey but still I need to see what's b开发者_JAVA技巧elow it.

What's an easy cross-browser solution for semi-transparency ? (Possibly without using images)

thanks

selector {
    filter: alpha(opacity=50); /* internet explorer /
    -khtml-opacity: 0.5;      / khtml, old safari /
    -moz-opacity: 0.5;       / mozilla, netscape /
    opacity: 0.5;           / fx, safari, opera */
}

http://perishablepress.com/press/2009/01/27/cross-browser-transparency-via-css/

CSS Transparency Settings for All Browsers
http://css-tricks.com/css-transparency-settings-for-all-broswers/