The below are all of the same but, each work for a certain browser. Besides the last one which is the standard CSS opacity property.

/* Applies to IE5-7 */
filter:alpha(opacity=85);

/* Applies to IE8 */
-ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";

/* Applies to Mozilla and Firefox */
-moz-opacity:0.85;

/* Applies to Safari 1.1-1.3 */
-khtml-opacity: 0.85;

/* Standard CSS property */
opacity: 0.85;

Since current browsers do not adopt to the same standards, developers will use the numerous CSS properties to create an equal cross-browser opacity experience, by placing each of those properties within there CSS.

The opacity property is supported by Netscape 7.2+, Firefox, Safari 2+, Opera 9+. The values that this property can accept are: 0.0 – 1.0. The lower the value the more transparent the element is.

The filter:alpha(opacity) property can range anywhere from 0-100. And like the the property above, the lower the value the more transparent.

The -khtml-opacity property was built with Webkit and it is based on the KHTML engine and associated the -khtml prefix to the -opacity

The -moz-opacity property applies for older versions of Mozilla browsers.

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