<style type="text/css">
    .Box{
        position: relative;
        width: 200px;
        height: 200px;
        background-color: red;
    }
    .Shade{
        position: absolute;
        top: 0;
        width: 100%;
        height: 100%;
        background-color: #333;
        opacity: 0.3;
    }
</style>

<div class="Box">
    <div class="Shade"></div>
    <div>
        <p>被遮罩元素</p>
        <button type="button">OK</button>
        <p>被遮罩元素</p>
    </div>
</div>