#box{
    background-color:rgb(125,25,0);
    opacity: 0.5
}

#box{
    background-color:rgb(125,25,0);
    opacity: 0.5;
    filter:alpha(opacity=50); /* 针对 IE8 以及更早的版本 */
}

#box{
    background-color: rgba(0, 0, 0, .5);
}

#box{
    filter: progid:DXImageTransform.Microsoft.Gradient(startColorStr=#80000000,endColorStr=#80000000);
}

<div id="box1">
    <div class="pane"></div>
    box1-opacity演示效果
</div>
<div id="box2">
    <div class="pane"></div>
    box2-rgba演示效果
</div>

#box1{
    width: 100px;
    height: 100px;
    color:black;
    background-color:rgb(125,25,0);
    opacity: 0.5;
}
#box2{
    margin-top: 10px;
    width: 100px;
    height: 100px;
    color:black;
    background-color: rgba(125,25,0, .5);
}
.pane{
    width: 20px;
    height: 20px;
    background-color:red;
}