CSS实现背景图片透明而文字不透明效果的两种方

.demo1{
    width: 500px;
    height: 300px;
    line-height: 50px;
    text-align: center;
}
.demo1:before{
    background: url(http://csssecrets.io/images/tiger.jpg) no-repeat;
    background-size: cover;
    width: 500px;
    height: 300px;
    content: "";
    position: absolute;
    : 0;
    left: 0;
    z-index: -1;/-1 可以当背景/
    -webkit-filter: blur(3px);
    filter: blur(3px);
}

<div class="demo1">背景图半透明，文字不透明<br />方法背景图+ filterblur</div>

.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;
    : 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<br />方法定位+ background:rgba(255,255,255,0.3)</div>
</div>