/* ie8 */
-ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=60)";

/* ie  5/7*/
filter:alpha(opacity=60);    

/* old mozilla browser like netscape  */
-moz-opacity:0.6; 

 /* for really really old safari */
-khtml-opacity: 0.6;    

/* css standard, currently it works in most modern browsers like firefox,  */
opacity: 0.8;    

Falls jQuery bereits eingebunden ist, schauen die Transparenzübergänge mittels jQuery aber besser aus.

$('#clickme').click(function() {
    $('#book').fadeTo('slow', 0.5, function() {
      // Animation complete.
    });
  });

[jQuery API]

“This script makes these CSS properties work transparently in IE version 5.0 upwards on all platforms except Mac”

.minheight-500 {
   min-height: 500px;
   height: auto !important; /* für moderne Browser */
   height:500px;  /*für den IE */
}

Normal comment: <!-- Comment text -->
Conditional comment: <!--[If expression]> HTML <![endif]-->

Für den einfachsten Switch bedeutet dies:
<!--[If IE]> HTML <![endif]-->
Hier wird jeder IE (ab v5) den eingeklammerten HTML code ausführen

Die umgekehrte Richtung geht leider nur mit einem nicht XHTML validen code, nämlich:
<!--[if !IE]><!--> HTML <!--<![endif]-->

Das Ganze geht auch komplexer:

• ! (negation)
  Use the negation to select all versions except the one specified, e.g.
  [If !IE 6] will select IE 5, 5.5 and 7.
• lt (less than)
  Select any versions less than the one specified, e.g. [If lt IE 6] will select IE 5 and 5.5.
• lte (less than or equal)
  e.g. [If lte IE 6] will select IE 5, 5.5 and 6.
• gt (greater than)
  Select any versions greater than the one specified, e.g. [If gt IE 6] will select IE 7 (and any later versions that may appear).
• gte (greater than or equal)
  e.g. [If gte IE 6] will select IE 6 and 7 (and later).

Für weitere Informationen [Quelle]