body,
html {
    height: 100%;
    margin: 0;
    overflow: hidden;
    background: black;
}

.firefly {
    position: relative;
    width: 100%;
    height: 100%;
}

.firefly::before,
.firefly::after {
    content: '';
    position: absolute;
    top: 50%;
    left: 50%;
    width: 10px;
    height: 10px;
    background: yellow;
    border-radius: 50%;
    box-shadow: 0 0 15px yellow, 0 0 30px yellow, 0 0 45px yellow;
    animation: fly 5s linear infinite;
}

.firefly::after {
    animation-delay: -2.5s;
}

@keyframes fly {

    0%,
    100% {
        transform: translate(-50%, -50%) translate(0, 0);
    }

    25% {
        transform: translate(-50%, -50%) translate(100px, -100px);
    }

    50% {
        transform: translate(-50%, -50%) translate(-100px, 100px);
    }

    75% {
        transform: translate(-50%, -50%) translate(100px, 100px);
    }
}