Animated backgrounds make your website livelier and more beautiful, like the code that fireflies are moving in the background of the website. Using this code on a dark background makes the movement of fireflies more natural at night.


HTML
<!-- This script got from www.devanswer.com  -->
<canvas id="canvas"></canvas><div id="bcl"><a style="font-size:8pt;text-decoration:none;" href="http://www.devanswer.com ">Developers Answer</a></div>
                        

CSS
body,
html {
    margin: 0px;
    padding: 0px;
    position: fixed;
    background-image: url('http://www.devanswer.com/img/image-back.png');
    background-size: cover;
}
canvas {
    filter: blur(1px);     
}
Javascript
let c = init("canvas"),
    w = (canvas.width = window.innerWidth),
    h = (canvas.height = window.innerHeight);
class firefly {
    constructor() {
        this.x = Math.random() * w;
        this.y = Math.random() * h;
        this.s = Math.random() * 2;
        this.ang = Math.random() * 2 * Math.PI;
        this.v = this.s * this.s / 4;
    }
    move() {
        this.x += this.v * Math.cos(this.ang);
        this.y += this.v * Math.sin(this.ang);
        this.ang += Math.random() * 20 * Math.PI / 180 - 10 * Math.PI / 180;
    }
    show() {
        c.beginPath();
        c.arc(this.x, this.y, this.s, 0, 2 * Math.PI);
        c.fillStyle = "#fddba3";
        c.fill();
    }
}
let f = [];
function draw() {
    if (f.length < 100) {
        for (let j = 0; j < 10; j++) {
            f.push(new firefly());
        }
    }
    //animation
    for (let i = 0; i < f.length; i++) {
        f[i].move();
        f[i].show();
        if (f[i].x < 0 || f[i].x > w || f[i].y < 0 || f[i].y > h) {
            f.splice(i, 1);
        }
    }
}
let mouse = {};
let last_mouse = {};
canvas.addEventListener(
    "mousemove",
    function (e) {
        last_mouse.x = mouse.x;
        last_mouse.y = mouse.y;

        mouse.x = e.pageX - this.offsetLeft;
        mouse.y = e.pageY - this.offsetTop;
    },
    false
);
function init(elemid) {
    let canvas = document.getElementById(elemid),
        c = canvas.getContext("2d"),
        w = (canvas.width = window.innerWidth),
        h = (canvas.height = window.innerHeight);
    c.fillStyle = "rgba(30,30,30,1)";
    c.fillRect(0, 0, w, h);
    return c;
}
window.requestAnimFrame = (function () {
    return (
        window.requestAnimationFrame ||
        window.webkitRequestAnimationFrame ||
        window.mozRequestAnimationFrame ||
        window.oRequestAnimationFrame ||
        window.msRequestAnimationFrame ||
        function (callback) {
            window.setTimeout(callback);
        }
    );
});
function loop() {
    window.requestAnimFrame(loop);
    c.clearRect(0, 0, w, h);
    draw();
}
window.addEventListener("resize", function () {
    (w = canvas.width = window.innerWidth),
        (h = canvas.height = window.innerHeight);
    loop();
});
loop();
setInterval(loop, 1000 / 60);