style.css

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

body {
    width: 100vw;
    height: 100vh;
    margin: 0;
    background-color: black;
    color: white;
    overflow: hidden;
    font-family: cursive;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    gap: 2vh;
}
h1,
h3 {
    margin: 0;
}
h1 {
    font-size: 10vh;
}
h3 {
    font-size: 3vh;
}
a {
    width: 40vh;
    height: 10vh;
    border: none;
    outline: none;
    color: #fff;
    background: #111;
    cursor: pointer;
    position: relative;
    z-index: 0;
    border-radius: 1rem;
    text-align: center;
    text-decoration: none;
    display: grid;
    place-items: center;
    font-size: 3vh;
}
a: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;
}
a:active:after {
    background: transparent;
}
a:hover:before {
    opacity: 1;
}
a: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;
    }
}