<style>
  #snowflakeContainer {
    position: absolute;
    left: 0px;
    top: 0px;
    display: none;
  }

  .snowflake {
    position: fixed;
    background-color: #CCC;
    user-select: none;
    z-index: 1000;
    pointer-events: none;
    border-radius: 50%;
    width: 10px;
    height: 10px;
  }
</style>

<div id="snowflakeContainer">
  <span class="snowflake"></span>
</div>

<script>
  // Array to store our Snowflake objects
  var snowflakes = [];

  // Global variables to store our browser's window size
  var browserWidth;
  var browserHeight;

  // Specify the number of snowflakes you want visible
  var numberOfSnowflakes = 50;

  // Flag to reset the position of the snowflakes
  var resetPosition = false;

  // Handle accessibility
  var enableAnimations = false;
  var reduceMotionQuery = matchMedia("(prefers-reduced-motion)");

  // Handle animation accessibility preferences 
  function setAccessibilityState() {
    if (reduceMotionQuery.matches) {
      enableAnimations = false;
    } else { 
      enableAnimations = true;
    }
  }
  setAccessibilityState();

  reduceMotionQuery.addListener(setAccessibilityState);

  //
  // It all starts here...
  //
  function setup() {
    if (enableAnimations) {
      window.addEventListener("DOMContentLoaded", generateSnowflakes, false);
      window.addEventListener("resize", setResetFlag, false);
    }
  }
  setup();

  //
  // Constructor for our Snowflake object
  //
  function Snowflake(element, speed, xPos, yPos) {
    // set initial snowflake properties
    this.element = element;
    this.speed = speed;
    this.xPos = xPos;
    this.yPos = yPos;
    this.scale = 1;

    // declare variables used for snowflake's motion
    this.counter = 0;
    this.sign = Math.random() < 0.5 ? 1 : -1;

    // setting an initial opacity and size for our snowflake
    this.element.style.opacity = (.1 + Math.random()) / 3;
  }

  //
  // The function responsible for actually moving our snowflake
  //
  Snowflake.prototype.update = function () {
    // using some trigonometry to determine our x and y position
    this.counter += this.speed / 5000;
    this.xPos += this.sign * this.speed * Math.cos(this.counter) / 40;
    this.yPos += Math.sin(this.counter) / 40 + this.speed / 30;
    this.scale = .5 + Math.abs(10 * Math.cos(this.counter) / 20);

    // setting our snowflake's position
    setTransform(Math.round(this.xPos), Math.round(this.yPos), this.scale, this.element);

    // if snowflake goes below the browser window, move it back to the top
    if (this.yPos > browserHeight) {
      this.yPos = -50;
    }
  }

  //
  // A performant way to set your snowflake's position and size
  //
  function setTransform(xPos, yPos, scale, el) {
    el.style.transform = `translate3d(${xPos}px, ${yPos}px, 0) scale(${scale}, ${scale})`;
  }

  //
  // The function responsible for creating the snowflake
  //
  function generateSnowflakes() {

    // get our snowflake element from the DOM and store it
    var originalSnowflake = document.querySelector(".snowflake");

    // access our snowflake element's parent container
    var snowflakeContainer = originalSnowflake.parentNode;
    snowflakeContainer.style.display = "block";

    // get our browser's size
    browserWidth = document.documentElement.clientWidth;
    browserHeight = document.documentElement.clientHeight;

    // create each individual snowflake
    for (var i = 0; i < numberOfSnowflakes; i++) {

      // clone our original snowflake and add it to snowflakeContainer
      var snowflakeClone = originalSnowflake.cloneNode(true);
      snowflakeContainer.appendChild(snowflakeClone);

      // set our snowflake's initial position and related properties
      var initialXPos = getPosition(50, browserWidth);
      var initialYPos = getPosition(50, browserHeight);
      var speed = 5 + Math.random() * 40;

      // create our Snowflake object
      var snowflakeObject = new Snowflake(snowflakeClone,
        speed,
        initialXPos,
        initialYPos);
      snowflakes.push(snowflakeObject);
    }

    // remove the original snowflake because we no longer need it visible
    snowflakeContainer.removeChild(originalSnowflake);

    moveSnowflakes();
  }

  //
  // Responsible for moving each snowflake by calling its update function
  //
  function moveSnowflakes() {

    if (enableAnimations) {
      for (var i = 0; i < snowflakes.length; i++) {
        var snowflake = snowflakes[i];
        snowflake.update();
      }      
    }

    // Reset the position of all the snowflakes to a new value
    if (resetPosition) {
      browserWidth = document.documentElement.clientWidth;
      browserHeight = document.documentElement.clientHeight;

      for (var i = 0; i < snowflakes.length; i++) {
        var snowflake = snowflakes[i];

        snowflake.xPos = getPosition(50, browserWidth);
        snowflake.yPos = getPosition(50, browserHeight);
      }

      resetPosition = false;
    }

    requestAnimationFrame(moveSnowflakes);
  }

  //
  // This function returns a number between (maximum - offset) and (maximum + offset)
  //
  function getPosition(offset, size) {
    return Math.round(-1 * offset + Math.random() * (size + 2 * offset));
  }

  //
  // Trigger a reset of all the snowflakes' positions
  //
  function setResetFlag(e) {
    resetPosition = true;
  }
</script>