Button.module.css

.glowOnHover {
    width: 220px; height: 50px;
    border: none; outline: none;
    color: #fff; background: #111;
    cursor: pointer; position: relative;
    z-index: 0; border-radius: 10px;
}
.glowOnHover: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 .3s ease-in-out;
    border-radius: 10px;
}
.glowOnHover:active {
    color: #000
}
.glowOnHover:active:after {
    background: transparent;
}
.glowOnHover:hover:before {
    opacity: 1;
}
.glowOnHover: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; }
}