div.a {
            width: 300px;
            text-align: center;
            color: white;
            background: black;
            font-weight: bold;
            font-size: 30px;
            position: relative; 
        }
        a.cover {
            width: 100%;
            height: 100%;
            position: absolute;
            top: 0;
            left: 0;
            background: linear-gradient(to bottom,black 0%,transparent 50%,black);
            opacity: 0.5;
        }

<div class="a">
        金属质感文字
        <a class="cover"></a>
    </div>