Edit in JSFiddle

<div class="box">
    <div class="t1">利用opacity，文字也跟着透明了</div>
    <div class="t2">这才是完美的背景透明</div>
</div>

.box {
    background: lightblue;
    padding: 10px;
}
.t1, .t2 {
    padding: 10px;
    margin: 10px;
    font-size: 24px;
}
.t1 {
    background: #fff;
    opacity: 0.2;
    filter: progid:DXImageTransform.Microsoft.Alpha(Opacity=20);
    -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=20)";
}

.t2 {
    background: rgb(255, 255, 255);
    background: rgba(255, 255, 255, 0.2);
    background: transparent\9;
    zoom: 1;
    filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#33ffffff, endColorstr=#33ffffff);
    -ms-filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#33ffffff, endColorstr=#33ffffff)";
}