为什么标签设置透明度会影响其他标签？

Question

<style type="text/css">.header{    height: 40px;    width:100%;    position: fixed;}.header_bg{    height: 40px;    width:100%;    opacity: 0.3;    background:#000;}.header .nav{    display: inline-block;    margin-top: -40px;    float: right;}.header .nav a{    padding-right: 20px;    color:#fff;    font-size: 14px;    line-height: 40px;}</style><div class="header">    <div>        <div class="header_bg"></div>    </div>    <div class="nav">        <a href="#">网页</a>        <a href="#">新闻</a>        <a href="#">贴吧</a>    </div></div>

Answer 1

影响了什么..