The styles applied to a Div Container to give it some opacity.
#mainContent {
width: 940px;
margin: 10px auto 50px auto;
padding: 30px;
border: 1px solid black;
color: black;
background-color:white;
/* for IE */
filter:alpha(opacity=45);
-moz-opacity:0.45;
/* CSS3 standard */
opacity: 0.45
}
Same method can be applied to a table background - sample here
Tested in IE6, IE7, IE6, Safari, Firefox
Reference site: http://www.w3schools.com/Css/css_image_transparency.asp