文字渐变

.a{
     position: fixed;
    top: 30px;
    width: 100%;
    height: 15px;
    margin-top: 10px;
    z-index: 10;
    background: -webkit-linear-gradient(#ffffff 50%, rgba(255, 255, 255, 0));
    background: linear-gradient(#ffffff 50%, rgba(255, 255, 255, 0));
}