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HTML 

              
                h1 Step by step form
h2 Just practising Javascript

form
  ul.items
  
  fieldset.username.enable
    div.icon.left: i.user
    input(type="text", name="username", placeholder="Username")
    div.icon.right.button: i.arrow

  fieldset.email
    div.icon.left: i.letter
    input(type="mail", name="email", placeholder="Email")
    div.icon.right.button: i.arrow

  fieldset.password
    div.icon.left: i.lock
    input(type="password", name="password", placeholder="Password")
    div.icon.right.button: i.arrow

  fieldset.thanks
    div.icon.left: i.heart
    p Thanks for your time
    div.icon.right: i.heart
!

CSS 

              
                body
  background: hsl(120, 60, 50)
  font-family: sans-serif
  fieldset
    box-shadow: 0 8px 10px hsl(120, 60, 40)
body.error
  background: hsl(16, 100, 47)
  fieldset
    box-shadow: 0 8px 10px hsl(16, 100, 37)
    
h1, h2
  position: absolute
  left: 50%
  transform: translateX(-50%)
  font-family: sans-serif
  text-transform: uppercase
  letter-spacing: 2px
h1
  top: 24px
  color: hsl(0, 0, 100)
  font-size: 12px
h2
  top: 44px
  color: hsl(0, 0, 100)
  font-size: 10px
  opacity: 0.7
  
ul.items
  position: absolute
  width: 30px
  height: auto
  top: 50%
  left: -60px
  transform: translateY(-50%)
  li
    width: 8px
    height: 8px
    margin: 10px 0
    background: hsl(0, 0, 100)
    border-radius: 50%
    opacity: 0.4
    cursor: pointer
  li.active
    opacity: 1
  
form
  position: absolute
  width: 300px
  height: 60px
  top: 50%
  left: 50%
  transform: translate(-50%, -50%)
  fieldset
    position: absolute
    width: 300px
    height: 60px
    background: hsl(0, 0, 100)
    border-radius: 3px
    opacity: 0
    transform: scale(0.2)
    transition: all 0.4s ease-in-out
    input, p
      display: inline-block
      width: 200px
      margin-left: 50px
      color: hsl(0, 0, 20)
      font-size: 16px
      letter-spacing: 1px
    p 
      margin-top: 22px
      text-align: center
    input
      height: 40px
      margin-top: 8px
      border: none
      outline: none
    .icon
      position: absolute
      width: 30px
      height: 30px
      top: 15px
      transition: all 0.4s ease
      i
        position: absolute
        display: block
        &::before, &::after
          position: absolute
          content: ''
    .icon.left
      left: 10px
    .icon.right
      right: 10px
      cursor: pointer
    .icon.button:hover
      background: hsl(0, 0, 95)
      border-radius: 3px
      transition: all 0.4s ease
        
  fieldset.enable
    z-index: 1
    opacity: 1
    transition: all 0.5s ease-out 0.2s
    transform: scale(1)
    animation: enable 0.5s ease-out 0.2s
  fieldset.disable
    opacity: 0
    transition: all 0.3s ease-in
    transform: translateY(120px) scale(0.9)
body.error fieldset
  transform-origin: 50% 100%
  animation: error 0.3s ease-out
    
@keyframes enable
  0%
    opacity: 0
    transform: scale(0.2)
  60%
    transform: scale(1.1)
  100%
    opacity: 1
    transform: scale(1)
@keyframes error
  0%, 50%, 100%
    transform: rotate(0deg)
  25%
    transform: rotate(-3deg)
  75%
    transform: rotate(3deg)
  
  
/**
 * Icons in CSS, long as f****
 */
.icon .arrow
  width: 2px
  height: 17px
  top: 5px
  left: 14px
  background: hsl(0, 0, 20)
  &::before
    width: 6px
    height: 6px
    bottom: -1px
    left: -3px
    border-color: hsl(0, 0, 20)
    border-right: 2px solid
    border-bottom: 2px solid
    transform: rotate(45deg)
.icon .user
  width: 20px
  height: 10px
  bottom: 5px
  left: 5px
  box-shadow: 0 0 0 2px hsl(0, 0, 20) inset
  border-radius: 6px 6px 3px 3px
  &::before
    width: 10px
    height: 10px
    top: -9px
    left: 5px
    box-shadow: 0 0 0 2px hsl(0, 0, 20) inset
    border-radius: 50%
.icon .letter
  width: 20px
  height: 12px
  top: 9px
  left: 5px
  box-shadow: 0 0 0 2px hsl(0, 0, 20) inset
  border-radius: 3px
  &::before, &::after
    width: 11px
    height: 2px
    top: 4px
    background: hsl(0, 0, 20)
  &::before
    left: 0
    transform: rotate(30deg)
  &::after
    right: 0
    transform: rotate(-30deg)
.icon .lock
  width: 20px
  height: 16px
  top: 9px
  left: 5px
  box-shadow: 0 0 0 2px hsl(0, 0, 20) inset
  border-radius: 3px
  &::before
    width: 8px
    height: 8px
    top: -4px
    left: 4px
    border: 2px solid transparent
    border-top: 2px solid hsl(0, 0, 20)
    border-right: 2px solid hsl(0, 0, 20)
    border-radius: 50%
    transform: rotate(-45deg)
  &::after
    width: 6px
    height: 7px
    top: 4px
    left: 7px
    box-shadow: 0 0 0 2px hsl(0, 0, 20) inset
.icon .heart
  width: 10px
  height: 10px
  top: 11px
  left: 7px
  background: hsl(9, 100, 60)
  transform: rotate(45deg)
  &::before, &::after
    width: 10px
    height: 10px
    border-radius: 50%
    background: hsl(9, 100, 60)
  &::before
    left: -6px
  &::after
    top: -6px
!

JS 

              
                /**
 * Surely there's some improvements to make, comments/help always appreciated :3
 */

function init() {
  // Generate li foreach fieldset
  for (var i = 0; i < count; i++) {
    var ul = document.querySelector('ul.items'),
        li = document.createElement("li");

    ul.appendChild(li);
  }
  // Add class active on first li
  ul.firstChild.classList.add('active');
}

function next(target) {
  var input = target.previousElementSibling;
  
  // Check if input is empty
  if (input.value === '') {
    body.classList.add('error');
  } else {
    body.classList.remove('error');
    
    var enable = document.querySelector('form fieldset.enable'),
        nextEnable = enable.nextElementSibling;
    enable.classList.remove('enable');
    enable.classList.add('disable');
    nextEnable.classList.add('enable');
    
    // Switch active class on left list
    var active = document.querySelector('ul.items li.active'),
        nextActive = active.nextElementSibling;
    active.classList.remove('active');
    nextActive.classList.add('active');
  }
}

function keyDown(event) {
  var key = event.keyCode,
      target = document.querySelector('fieldset.enable .button');
  if (key == 13 || key == 9) next(target);
}

var body = document.querySelector('body'),
    form = document.querySelector('form'),
    count = form.querySelectorAll('fieldset').length;

window.onload = init;
document.body.onmouseup = function (event) {
    var target = event.target || event.toElement;
    if (target.classList.contains("button")) next(target);
};
document.addEventListener("keydown", keyDown, false);
!
999px

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