<main>
  <h2>CSS Animations: the <code>animation-duration</code> Property</h2>

  <p>Use the inputs below to set a duration for the animation. You can choose to define the duration using seconds or milliseconds.</p>

  <div class="box">
  </div>
  
  <label for="duration">Set a duration:</label> <input id="duration" type="number" value="1" min="0" step="0.1">
  <select id="unit">
    <option value="s">s</option>
    <option value="ms">ms</option>
  </select>

  <br>

  <button>Animate the Box</button> <button disabled>Reset</button>

</main>

body {
  font-family: Arial, sans-serif;
  font-size: 20px;
  padding: 0 20px;
}

main {
  text-align: center;
  margin: 0 auto;
  max-width: 800px;
}

p {
  text-align: left;
  padding: 0 20px;
}

code {
  color: firebrick;
}

.box {
  width: 100px;
  height: 100px;
  background: lightgreen;
  border: solid 2px;
  margin-bottom: 20px;
}

.box-animate {
  animation-name: moveObject;
  animation-timing-function: linear;
}

@keyframes moveObject {
  from {
    transform: translateX(0);
  }

  50% {
    transform: translateX(300px);
  }

  to {
    transform: translateX(300px) scale(1.5);
  }
}

input[type=number] {
  width: 100px;  
}

button {
  margin-top: 20px;
}

let myObject = document.querySelector('.box'),
    btn1 = document.querySelector('button'),
    btn2 = document.querySelectorAll('button')[1];

btn1.addEventListener('click', function () {
  myObject.style.animationDuration = document.querySelector('#duration').value + document.querySelector('#unit').value;
  myObject.classList.add('box-animate');
  this.disabled = true;
  btn2.disabled = false;
}, false);

btn2.addEventListener('click', function () {
  myObject.classList.remove('box-animate');
  this.disabled = true;
  btn1.disabled = false;
}, false);