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HTML 

              
                <!-- Mobile container -->
<main class="container">
    <!-- Progress bar -->
    <div class="progress"></div>
    <!-- Countdown counter -->
    <div class="counter">1</div>
    <!-- Button to toggle countdown -->
    <button class="button">Toggle</button>
</main>
!

CSS 

              
                :root {
  // CSS variable to update progress
  --progress: 100;
}

*, *:before, *:after {
  box-sizing: border-box;
}

body {
  overflow: hidden;

  // Decorative background
  &:before {
    content: '';
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    border-left: 50vw solid #FF6E6E;
    border-right: 50vw solid #FF6E6E;
    border-top: 50vh solid #354051;
    border-bottom: 50vh solid #354051;
    z-index: -1;
  }
}

// Mobile container
.container {
  position: absolute;
  left: 50%;
  top: 50%;
  width: 340px;
  height: 540px;
  background-color: #354051;
  box-shadow: 0 0 100px rgba(0, 0, 0, 0.5);
  transform: translate(-50%, -50%);
  overflow: hidden;

  // Header bar, just a aesthetic detail
  &:before {
    content: '';
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 25px;
    background-color: rgba(0, 0, 0, 0.2);
    z-index: 3;
  }
}

// Container for progress bar and expansive button
.progress {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  z-index: 1;
  pointer-events: none;

  // Expansive button, initially the red button we can see
  &:before {
    content: '';
    position: absolute;
    left: calc(50% - 30px);
    bottom: 50px;
    width: 60px;
    height: 60px;
    background-color: #FF6E6E;
    border-radius: 100%;
    transform: scale(1);
    transition: 0.8s transform;
  }

  // Progress bar
  &:after {
    content: '';
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(0, 0, 0, 0.1);
    box-shadow: 0 50px 0 50px rgba(0, 0, 0, 0.1);
    transform: translateY(calc(var(--progress) * 1%)); // Set translation using `--progress`
    transition: 0.95s transform cubic-bezier(0.175, 0.885, 0.320, 1.275); // easeOutBack
  }
}

// Countdown counter, initially hidden
.counter {
  display: flex;
  align-items: center;
  justify-content: center;
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  font-size: 100px;
  font-family: 'Roboto Mono', monospace;
  color: #FFFFFF;
  pointer-events: none;
  opacity: 0;
  z-index: 3;
  transition: 0.5s opacity;
}

// Button to toggle countdown
.button {
  position: absolute;
  left: calc(50% - 30px);
  bottom: 50px;
  width: 60px;
  height: 60px;
  padding: 0;
  color: transparent;
  background-color: transparent;
  outline: none;
  border: none;
  border-radius: 100%;
  text-indent: -999px; // Hiding text
  z-index: 2;
  cursor: pointer;

  // Gray button to stop countdown
  &:before {
    content: '';
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    border-radius: 100%;
    background-color: transparent;
    transform: scale(0);
    transition: 0.5s 0.5s transform, 0.5s 0.5s background-color;
  }

  // Element for the expansive shadow effect on click
  &:after {
    content: '';
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    border-radius: 100%;
    opacity: 0;
    box-shadow: inset 0 0 0 20px rgba(255, 255, 255, 1);
    transition: 0s;
    pointer-events: none;
    z-index: -1;
  }
}

// Styles for countdown running
.container--running {

  // Expand the red button to cover the container entirely
  // Using the `easeInBack` easing to get a nice effect
  .progress:before {
    transform: scale(17);
    transition: 0.8s transform cubic-bezier(0.600, -0.280, 0.735, 0.045); // easeInBack
  }

  // Show the counter
  .counter {
    opacity: 1;
    transition: 1s opacity;
  }

  .button {

    // Show the gray circle (stop button)
    &:before {
      background-color: #354051;
      transform: scale(1);
      transition: 0.5s 0.5s transform, 0.5s 0.5s background-color;
    }

    // Scale the element and reduce the shadow, getting the desired effect
    &:after {
      opacity: 1;
      transform: scale(2.5);
      box-shadow: inset 0 0 0 0 rgba(255, 255, 255, 0);
      transition: all 0.8s, opacity 0.3s;
    }
  }
}


// _ms-countdown-timer.scss

// Slider dimmensions
$ms-container-width: 340px;
$ms-slide-width: 125px;
$ms-slide-height: 540px;

// Slider container
.ms-container {
  position: relative;
  top: 50%;
  width: $ms-container-width;
  max-width: 100%;
  margin: 0 auto;
  overflow: hidden;
  transform: translateY(-50%);

  // Gradients to hide the adjacent numbers smoothly
  &:before, &:after {
    content: '';
    position: absolute;
    top: 0;
    width: 80px;
    height: 100%;
    z-index: 1;
    pointer-events: none;
  }
  &:before {
    left: 0;
    background-image: linear-gradient(to right, rgba(53, 64, 81, 1) 25%, rgba(53, 64, 81, 0));
  }
  &:after {
    right: 0;
    background-image: linear-gradient(to left, rgba(53, 64, 81, 1) 25%, rgba(53, 64, 81, 0));
  }
}

// Slider track
.ms-track {
  position: relative;
  left: calc(50% - #{$ms-slide-width / 2}); // Centering
  white-space: nowrap;
  font-size: 0;
  list-style: none;
  padding: 0;
  margin: 0;
  will-change: transform;
}

// Slides
.ms-slide {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: $ms-slide-width;
  height: $ms-slide-height;
  font-size: 100px;
  font-family: 'Roboto Mono', monospace;
  color: #FFFFFF;
  cursor: default;
  user-select: none;
  will-change: transform;
}
!

JS 

              
                (function() {

    // Variables to use later
    var root = document.documentElement;
    var container = document.querySelector('.container');
    var button = document.querySelector('.button');
    var counter = document.querySelector('.counter');
    var running = false;
    var timer = null;
    var seconds = 0;
    var secondsInitial = 0;

    // Initializing the slider
    var ms = new MomentumSlider({
        el: container,  // HTML element to append the slider
        range: [1, 60], // Generar los elementos del slider usando el rango de números definido
        loop: 2,        // Hacer infinito el slider, añadiendo 2 elementos extra en cada extremo
        style: {
            // Estilos para interpolar
            // El primer valor corresponde a los elementos adyacentes
            // El segundo valor corresponde al elemento actual
            transform: [{scale: [0.4, 1]}],
            opacity: [0.3, 1]
        }
    });

    // Simple toggle functionality
    button.addEventListener('click', function () {
        if (running) {
            stop();
        } else {
            start();
        }
        running = !running;
    });

    // Start the countdown
    function start() {
        // Disable the slider during countdown
        ms.disable();
        // Get current slide index, and set initial values
        seconds = ms.getCurrentIndex() + 1;
        counter.innerText = secondsInitial = seconds;
        root.style.setProperty('--progress', 0);
        // Add class to trigger CSS transitions for `running` state
        container.classList.add('container--running');
        // Set interval to update the component every second
        timer = setInterval(function () {
            // Update values
            counter.innerText = --seconds;
            root.style.setProperty('--progress', (secondsInitial - seconds) / secondsInitial * 100);
            // Stop countdown if it's finished
            if (!seconds) {
                stop();
                running = false;
            }
        }, 1000);
    }

    // Stop the countdown
    function stop() {
        // Enable slider
        ms.enable();
        // Clear interval
        clearInterval(timer);
        // Reset progress
        root.style.setProperty('--progress', 100);
        // Remove `running` state
        container.classList.remove('container--running');
    }

})();
!
999px

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