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POPSCSS Opacity in Mozilla, IE, and Opera Opacity works in Mozilla and IE. It also works in current versions of Opera. The style filter: alpha(opacity=50); for IE and the style -moz-opacity: 0.5; for Mozilla (releases earlier than 2004) parallel the w3.org CSS3 specification for opacity opacity:.50; . Note that -moz-opacity: 0.5; isn't needed in current versions of Mozilla Firefox.