背景图片透明，文字不透明效果

.demo2-bg{
    background: url(http://csssecrets.io/images/tiger.jpg) no-repeat;
    background-size: cover;
    width: 500px;
    height: 300px;
    position: relative;
}

.demo2{
    position: absolute;
    left: 0;
    right: 0;
    top: 0;
    bottom: 0;
    width: 500px;
    height: 300px;
    line-height: 50px;
    text-align: center;
    background:rgba(255,255,255,0.3);
}


<div class="demo2-bg">
    <div class="demo2">背景图半透明，文字不透明</div>
</div>