body {
    margin: 0;
    padding: 0;
    background-color: black;
}

a {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    width: 200px;
    height: 60px;
    text-align: center;
    line-height: 60px;
    color: #fff;
    font-size: 24px;
    text-decoration: none;
    font-family: sans-serif;
    background: linear-gradient(90deg, #03a9f4, #f441a5, #ffeb3b, #03a9f4);
    border-radius: 30px;
    z-index: 1;

    /* Below */
    background-size: 400%;
}

a:hover {
    animation: animate 8s linear infinite;
}

a::before {
    content: '';
    position: absolute;
    inset: -5px;
    z-index: -1;
    background: linear-gradient(90deg, #03a9f4, #f441a5, #ffeb3b, #03a9f4);
    background-size: 400%;
    border-radius: 40px;
    opacity: 0;
}

a:hover::before {
    filter: blur(20px);
    opacity: 1;
    animation: animate 8s linear infinite;
}

@keyframes animate {
    from {
        background-position: 0;
    }
    to {
        background-position: 400%;
    }
}