IE：
filter:alpha(opacity=sqlN)
其中 sqlN的值域为[0, 100]
js: ieNode.style.filter="alpha(opacity=sqlN)";
Firefox,Chrome和Safari：
opacity:sqlN
其中sqlN的值域为[0, 1]
js: firefoxNode.style.opacity=sqlN;
测试代码如下：
<HTML> <HEAD> <style type="text/css"> .ieCss { display:-moz-inline-box; display:inline-block; width:100; height:100; background-color:red; filter:alpha(opacity=20); } .fireFoxCss { display:-moz-inline-box; display:inline-block; width:100; height:100; background-color:blue; opacity:0.5; } </style> <script> window.onload = function() { //设置IE var ieSpanJs = document.getElementById("ieSpanJs"); ieSpanJs.style.display = "inline-block"; //ie支持 ieSpanJs.style.width = 100; ieSpanJs.style.height = 100; ieSpanJs.style.backgroundColor = "red"; ieSpanJs.style.filter="alpha(opacity=20)"; //设置firefox var firefoxSpanJs = document.getElementById("firefoxSpanJs"); try { firefoxSpanJs.style.display = "-moz-inline-box"; //firefox支持 } catch (e) { firefoxSpanJs.style.display = "inline-block"; //支持IE } firefoxSpanJs.style.width = 100; firefoxSpanJs.style.height = 100; firefoxSpanJs.style.backgroundColor = "blue"; firefoxSpanJs.style.opacity="0.5"; } </script> </HEAD> <BODY> <span id="ieSpanCss" class="ieCss">IE_css</span> <span id="firefoxSpanCss" class="fireFoxCss">firefox_css</span> <br> <br> <span id="ieSpanJs">IE_Js</span> <span id="firefoxSpanJs">firefox_Js</span> </BODY> </HTML>
补充：参考资料中对firfox还有3.5之前和3.5之后版本的区分，暂时忽略。
参考文档：