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HTML

              
                <div class="one"></div>
              
            
!

CSS

              
                @import 'compass';

// Update 2020: Particle number has been reduced due to time-out errors
$num: 500; // Number of particles (<2000) Gif is 1900
$sat: 90; // Color saturation
$light: 60; // Color lightness
$dur: 3.5s; // Animation duration

/* Shape at start of animation */
// Play with these options...
$startOptions: (
  size: 3px,
  width: 30,
  height: 30,
  
  loops: 3,
  twists: 1,

  minHue: 0,
  maxHue: 360
);

/* Shape at end of animation */
// ... and these options
$endOptions: (
  size: 3px,
  width: 0.125,
  height: 0.125,
  
  loops: 12,
  twists: 16,
  
  minHue: 0,
  maxHue: 360
);

/* Shortcut functions to call options from maps above */
@function s($option){
  @return map-get($startOptions,$option);
}

@function e($option){
  @return map-get($endOptions,$option);
}

/* Set color based on min/max hue and current shadow */
@function hueRange($shadow, $hue) {
  $minHue: call($shadow, minHue);
  $maxHue: call($shadow, maxHue);
	@return $hue / $num * ($maxHue - $minHue) + $minHue;
}

/* Lists to hold shadows */
$start-shadow: ();
$end-shadow: ();

// Build box-shadow lists
@for $n from 0 through $num {
  $start-shadow: append($start-shadow, 
    sin($n  // position x
      * s(loops)) 
      * s(size) 
      * s(width)
    
    cos($n  // position y
      * s(twists)) 
      * s(size) 
      * s(height)
    
    0 // blur
    
    hsl(hueRange(s,$n),$sat,$light) // color 
    
    , comma); 

  // End shadow, same structure as above but with cos/sine reversed
  $end-shadow: append($end-shadow, cos($n * e(loops)) * e(size) * $n * e(width)
    sin($n * e(twists)) * e(size) * $n * e(height) 0 hsl(hueRange(e,$n),$sat,$light), comma);
}

// Body styling
body {
  background: hsl(230,70,10);
}

.one {
  // Div styling
  background: transparent;
  width: s(size);
  height: s(size);
  border-radius: 50%;
  
  // Center div
  position: absolute;
    top: 50%;
    left: 50%;
  margin-top: s(size);
  margin-left: s(size);
  
  // Magic
  animation: anim $dur infinite alternate ease-in-out;
  box-shadow: $start-shadow;
}

@keyframes anim {
  5% {
    box-shadow: $start-shadow;
  }
  95%,100% {
    box-shadow: $end-shadow;
    width: e(size);
    height: e(size);
  }
}
              
            
!

JS

              
                // v1: https://codepen.io/giana/pen/JGdbje/
              
            
!
999px

Console