<canvas id=c></canvas>
<canvas id=bg></canvas>
body {
  background-color:black;
}
#c {
  opacity:.8;
}
canvas {
  position:absolute;
  top:0; left:0;
}
    //initial
var w = c.width = window.innerWidth,
    h = c.height = window.innerHeight,
    ctx = c.getContext('2d'),
    
    //parameters
    total = w,
    accelleration = .05,
    
    //afterinitial calculations
    size = w/total,
    occupation = w/total,
    repaintColor = 'rgba(0, 0, 0, .04)'
    colors = [],
    dots = [],
    dotsVel = [];

//setting the colors' hue
//and y level for all dots
var portion = 360/total;
for(var i = 0; i < total; ++i){
  colors[i] = portion * i;
  
  dots[i] = h;
  dotsVel[i] = 10;
}

function anim(){
  window.requestAnimationFrame(anim);
  
  ctx.fillStyle = repaintColor;
  ctx.fillRect(0, 0, w, h);
  
  for(var i = 0; i < total; ++i){
    var currentY = dots[i] - 1;
    dots[i] += dotsVel[i] += accelleration;
    
    ctx.fillStyle = 'hsl('+ colors[i] + ', 80%, 50%)';
    ctx.fillRect(occupation * i, currentY, size, dotsVel[i] + 1);
    
    if(dots[i] > h && Math.random() < .01){
      dots[i] = dotsVel[i] = 0;
    }
  }
}

anim();

External CSS

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External JavaScript

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