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This document is a work in progress and is subject to change.


  • Firefox allows you to use coding to manipulate the opacity of an image.
  • "Opacity" refers to transparency; the lesser the opacity of an image, the more transparent it is.

Initial Value:1(opaque)
Applies toall elements


Opacity:1.0 shows you all of the image; Without any filters applied the opacity of an image is 1.0 (fully opaque).

We use CSS to define the initial degree of opacity in Firefox:

<IMG style="-moz-opacity:0.5" src="test.gif">

Valid range of values for -moz.opacity is from 0 to 1, where lesser equates to more transparency.


  • Quick opacity change onMouseover:
<IMG style="-moz-opacity:0.6" src="alta.gif" onMouseover="this.style.MozOpacity=1" onMouseout="this.style.MozOpacity=0.6">
  • For CSS opacity to work in the browser it must have some placement specified. Mozilla recommends that we use float:left; and position:relative; to work in Mozilla Firefox. The opacity code for simply displaying an image is more straightforward.
<span style="float:left;filter:alpha(opacity=25);-moz-opacity:.25;opacity:.25;"><img src="v90.gif" width="50" height="50"></span>

Fun with -moz-opacity

To make your page more attractive, you can do something like this:

Fun stuff.png

<div style="color:black;width:150px;background:white;filter:alpha(opacity=50);-moz-opacity:.50;
font-family:Verdana, Arial, Helvetica,Georgia;font-size: 28px;line-height:26px;  text-align: right;">
...this will look like some of the  
<b>pullquotes or subtitles we see</b>
<span style="width:150px;padding-bottom:10px;float:right;text-align:left;color:black;background:white;

Mozilla Recommended Values

MDC document on -moz-opacity


The World Wide Web Consortium (W3C) provides a free online CSS validation service.

Browser Compatibility

  • Internet Explorer
  • Firefox
  • Opera