table cell semi transparency

I was surfing the web the other day and found a site that had the coolest effect on table cells. We've all seem the cell that change color when you scroll over them, but this was different. At first I thought they had done the tetious job of creating graphics for each rollover cell with a different shade, but when I looked into it I realized that it was not so. Some how they were manipulating the color to be semi transparent, changing the shade of the back ground image. I looked into the code but could not find anything out of the ordinary. Anyone have any ideas? If you know how to do this, let me know.

You can do that with CSS, but it only works in Internet Explorer. Take a look at that sites CSS to see how it works.