.loader-container {
  width:100px;
  height:100px;
  position: absolute;
  animation: scaleUp .5s linear;
  opacity: 0;
  transform: scale(0);
  .loader {
    background: #fff;
    animation: filterHue 2.5s linear infinite;
    height:100%;
    width: 100%;
  }
}
  @keyframes scaleUp {
  50% { transform: scale(0.5); opacity: 1; }
  100% { transform: scale(0.25) }
  }

window.addEventListener("mousemove", function (e) {
  var to_append = document.getElementsByClassName('loader-container')[0];
  var all = document.getElementsByClassName('loader-container');
  var parent_div = document.createElement('div');
  parent_div.className = "loader-container";
  var inner_div = document.createElement('div');
  inner_div.className = "loader";
  parent_div.appendChild(inner_div)
  var d = document.body.appendChild(parent_div);
  parent_div.style.left = (e.clientX - 50)+'px';
  parent_div.style.top = (e.clientY - 50)+'px';
  if(document.getElementsByClassName('loader-container').length > 50) {
    document.body.removeChild(to_append)
  }
});

<div id="timer">00:00:00</div>
<button onclick="startStopTimer()">Старт/Стоп</button>
<button button onclick="resetTimer()">Сброс</button>

body { font-family: Arial, sans-serif; text-align: center; margin: 50px; }
#timer { font-size: 2em; margin-bottom: 20px; }
button { font-size: 1em; padding: 10px 20px; cursor: pointer; }

let timer, s = 0, m = 0, h = 0, t = 0;
  function startStopTimer() {
    if (t) {
      clearInterval(timer);
    } else {
      timer = setInterval(() => {
        t ? s++ : clearInterval(timer);
        updateTimer();
      }, 1000);
    } t ^= 1;}
  function resetTimer() {
    clearInterval(timer);
    timer = undefined;
    s = m = h = 0;
    updateTimer();
    t = 0; }
  function updateTimer() {
    const padZero = (value) => (value < 10 ? '0' : '') + value;
    document.getElementById('timer').innerText = `${padZero(h)}:${padZero(m)}:${padZero(s)}`; }

<div id="clock"></div>

#clock { font-size: 50px; font-family: Arial, sans-serif; text-align: center; margin: 100px; }

function updateClock() {
    const now = new Date();
    const hours = now.getHours().toString().padStart(2, '0');
    const minutes = now.getMinutes().toString().padStart(2, '0');
    const seconds = now.getSeconds().toString().padStart(2, '0');
    document.getElementById('clock').innerText = `${hours}:${minutes}:${seconds}`;
  }
  // Обновляем часы каждую секунду
  setInterval(updateClock, 1000);
  // Запускаем обновление сразу, чтобы избежать задержки в отображении времени
  updateClock();

body {
    transition: background-color 0.5s ease;
    margin: 0;
    height: 100vh;
}

function changeBackground() {
  const randomColor = '#' + Math.floor(Math.random() * 16777215).toString(16);
  document.body.style.backgroundColor = randomColor;
}
document.body.addEventListener('click', changeBackground);