div透明但不導致內容div透明的解決方法

 將兩個層分開不嵌套即可

#div1{width:300px; height:200px;  position:relative；  }
. nav{ background:black;filter:alpha(opacity:30);opacity:0.3; height:100%;position:absolute;width:100%;top:0; z-index:1 }
.box { width:300px; height:20px; background:blue; position:absolute; bottom; top:0;z-index:0; }

<div id="div1">
    <div class="nav"></div>
    <div class="box">這裏是內容部分</div>
</div>