用來設定元素透明度的 Opacity 是CSS 3里的一個屬性。當然現在還只有少部分瀏覽器支持。不過各個瀏覽器都有自己的私有屬性來支持,其中包括老版本的Mozilla和Safari：

IE: filter:alpha(opacity)
Mozilla: -moz-opacity
Safari: -khtml-opacity

很不幸的是，你沒看見Opera，老版本的Opera并沒有什么私有屬性可以代替opacity。（新版Opera已經支持opacity）

所以以前用CSS設定一個元素半透明的話，可能會這樣寫:

CSS Code復制內容到剪貼板

.opacity{   
filter:alpha(opacity=50); /* IE */  
-moz-opacity:0.5; /* 老版Mozilla */  
-khtml-opacity:0.5; /* 老版Safari */  
opacity: 0.5; /* 支持opacity的瀏覽器*/  
}  

用javascript來設定一個元素為半透明:

JavaScript Code復制內容到剪貼板

object.filter = "alpha(opacity=" + opacity + ")"; /* IE */  
object.MozOpacity = (opacity / 100); /* 老版Mozilla */  
object.KhtmlOpacity = (opacity / 100); /* 老版Safari */  
object.opacity = (opacity / 100); /* 支持opacity的瀏覽器*/  
  

下面給大家介紹css透明度的設置 （兼容所有瀏覽器）

一句話搞定透明背景！

CSS Code復制內容到剪貼板

.transparent_class {          
      filter:alpha(opacity=50);          
      -moz-opacity:0.5;          
      -khtml-opacity: 0.5;          
      opacity: 0.5;          
}       

UPDATE: I wanted to pull this post out of the archives and update it a bit because it there seems to be a good amount of interest in this subject.

Here is what each of those CSS properties is for:

opacity: 0.5; This is the “most important” one because it is the current standard in CSS. This will work in most versions of Firefox, Safari, and Opera. This would be all you need if all browsers supported current standards. Which, of course, they don’t.
filter:alpha(opacity=50); This one you need for IE.
-moz-opacity:0.5; You need this one to support way old school versions of the Mozilla browsers like Netscape Navigator.
-khtml-opacity: 0.5; This is for way old versions of Safari (1.x) when the rendering engine it was using was still referred to as KTHML, as opposed to the current WebKit .