css 中在图片中加入文字的方法

<div class=‘a‘>
    <div class=‘b‘></div>
    <div class=‘c‘>
        <span>文字</span>
    </div>
    <div class=‘d‘></div>
</div>

    .a{
        width:400px;
        height:400px;
        position:relative;
    }

    .b{
        width: 100%;
        height:100%;
        background:orange;
    }

    .c span{
        display: block;
        padding-top: 40%;
        text-align: center;
        z-index: 5;
        position: absolute;
        top:0px;
        left: 0px;
        width: 100%;
        color: #fff;
    }

    .d{
        position: absolute;
        top:0px;
        left: 0px;
        width: 100%;
        height: 100%;
        background:black;
        opacity: 0.5;
        z-index: 3;
    }