.button {
    display: block;
    text-decoration: none;
    color: #fff;
    padding: 3px;
    border-radius: 6px;
    position: relative;
    overflow: hidden;
}

.button::before {
    content: "";
    position: absolute;
    top: 0;
    left: 0;
    width: 200%;
    height: 100%;
    background:
            linear-gradient(115deg,
            #000000,#373333,#808080);
    background-size: 50% 100%;
    border-radius: inherit;
    }

.button:hover::before {
    animation: animate_border .5s linear infinite;
}

@keyframes animate_border {
    to {
        transform: translate(-50%)
    }
}

.button span {
    display: block;
    background-color: black;
    padding: 5px;
    border-radius: 3px;
    position: relative;
    z-index: 2;
}