body, html {
  height: 100%;
   background-image: url("https://images.unsplash.com/photo-1483982258113-b72862e6cff6?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&w=1000&q=80");

  /* Center and scale the image nicely */
  background-position: center;
  background-repeat: no-repeat;
  background-size: cover;
  
  
  overflow: hidden;
}

* {
  box-sizing: border-box;
}
.firefly {
    width: 2px;
    height: 2px;
    background: yellow;
    border-radius: 50%;
    box-shadow: 0 0 0 5px rgba(154, 205, 50, 0.4);
    overflow: hidden;
}
const FIREFLIES = 10;
const maxHeight = 600;
const maxWidth = 1200;

const random = (min, max) => {
    return Math.floor(Math.random() * ( max - min ) + min);
}
for(var i = 0; i < FIREFLIES; i++) {
    let div = document.createElement('div');
    div.setAttribute('class', 'firefly');
    document.body.appendChild(div);
}
var fireflies = document.querySelectorAll('.firefly');
for(var i = 0; i < FIREFLIES; i++) {
    let ix = random(-10, maxWidth+10);
    let fx = random(-10, maxWidth+10);
    
    let iy = random(-10, maxHeight+10);
    let fy = random(-10, maxHeight+10);

    gsap.fromTo(fireflies[i], {
        x: ix,
        y: iy,
    }, {
        x: fx,
        y: fy,
        duration: random(5, 15),
        // ease: "slow (0.7, 0.7, false)",
        ease: "rough ({template: none.out, strength: 1, points: 20, taper: 'none', randomize: true, clamp:false})",
        repeat: -1,
        yoyo: true,
    });
    gsap.fromTo(fireflies[i], {
        boxShadow: "0 0 0 0px rgba(154, 205, 50, 0)"
    }, {
        boxShadow: "0 0 0 3px rgba(154, 205, 50, 0.4)",
        duration: random(2, 4),
        ease: "slow (0.7, 0.7, false)",
        repeat: -1,
        yoyo: true,
    });
    gsap.fromTo(fireflies[i], {
        opacity: 0,
    }, {
        duration: random(1, 5),
        opacity: 0.9,
        repeat: -1,
        yoyo: true,
    })
};

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/gsap/3.3.4/gsap.min.js