HTMLify

*,
*::before,
*::after {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

body {
    min-height: 100vh;
    display: grid;
    place-items: center;
    background: #000;
}

.rainbow_btn {
    width: 220px;
    height: 50px;
    border: none;
    outline: none;
    color: #fff;
    background: #111;
    cursor: pointer;
    position: relative;
    z-index: 0;
    border-radius: 10px;
}

.rainbow_btn::before {
    content: "";
    background: linear-gradient(45deg,
            #ff0000,
            #ff7300,
            #fffb00,
            #48ff00,
            #00ffd5,
            #002bff,
            #7a00ff,
            #ff00c8,
            #ff0000);
    position: absolute;
    top: -2px;
    left: -2px;
    background-size: 400%;
    z-index: -1;
    filter: blur(5px);
    width: calc(100% + 4px);
    height: calc(100% + 4px);
    animation: glowing 20s linear infinite;
    opacity: 0;
    transition: opacity 0.3s ease-in-out;
    border-radius: 10px;
}

.rainbow_btn:active {
    color: #000;
}

.rainbow_btn:active::after {
    background: transparent;
}

.rainbow_btn:hover::before {
    opacity: 1;
}

.rainbow_btn::after {
    z-index: -1;
    content: "";
    position: absolute;
    width: 100%;
    height: 100%;
    background: #111;
    left: 0;
    top: 0;
    border-radius: 10px;
}

@keyframes glowing {
    0% {
        background-position: 0 0;
    }

    50% {
        background-position: 400% 0;
    }

    100% {
        background-position: 0 0;
    }
}