已经不用了

浏览器优先级别 FF < IE7 < IE6

demo

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.box {
    /* safari/chrome */
    -webkit-opacity:.5;
    /* firefox 0.9 */
    -moz-opacity:.5;
    /* safari 1.x */
    -khtml-opacity:.5;
    /* ie4 - ie9 */
    filter:alpha(opacity=50);
    opacity:.5;
}

.demo {
  opacity: .5;
}
/* IE6 */
* html .demo {
  opacity: .5;
}
/* IE7 */
*+html .demo {
  opacity: .5;
}





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