body, html {
    margin: 0;
    padding: 0;
    height: 100%;
}

.gradient {
    width: 100%;
    height: 100vh;
    background: linear-gradient(to right, red, orange, yellow, green, blue, indigo, violet);
    background-size: 1400% 100%;
    animation: rainbow 15s linear infinite;
}

@keyframes rainbow {
    0% {
        background-position: 0% 50%;
    }
    50% {
        background-position: 100% 50%;
    }
    100% {
        background-position: 0% 50%;
    }
}