IE9, Firefox, Chrome, Opera 和 Safari 使用属性 opacity 来设定透明度。opacity 属性能够设置的值从 0.0 到 1.0。值越小，越透明。
IE8
以及更早的版本使用滤镜 filter:alpha(opacity=x)。x
能够取的值从 0 到 100。值越小，越透明。
<head> <style> div.background { width:
400px; height: 266px; background: url(‘/i/tulip_peach_blossom_w.jpg‘)
no-repeat;//不允许平铺 border: 1px solid black; } div.transbox { width: 338px;
height: 204px; margin:30px; background-color: #ffffff; border: 1px solid
black; /*
for IE */ filter:alpha(op
acity=60); /*
CSS3 standard */ opacity:0.6;//设置滤镜 } div.transbox p {
margin: 30px 40px;} </style> </head> <body> <div
class="background"> <div class="transbox"> <p>This is some text
that is placed in the transparent box.This is some text that is placed in the
transparent box.This is some text that is placed in the transparent box.This is
some text that is placed in the transparent box.This is some text that is placed
in the transparent box. </p> </div> </div> </body〉
原文：http://www.cnblogs.com/HorseKing/p/3550605.html